Add documentation for media controls pipeline

# SysUI Media Controls Pipeline

[TOC]

## Purpose

Describe how events flow through the media controls pipeline, and provide a high level overview of what the different components do.

## Pipeline Diagram

![media controls pipeline](media-controls-pipeline.png)

* Orange: External inputs

* Blue: Internal listeners; all except `MediaDataManager` and `ResumeMediaBrowser` implement [`MediaDataManager.Listener`](/packages/SystemUI/src/com/android/systemui/media/MediaDataManager.kt#711) and receive `onMediaDataLoaded` and `onMediaDataRemoved` events

## Classes

Files under [`systemui/media/`](/packages/SystemUI/src/com/android/systemui/media/):

* UI

   * `dialog/`

      * Output switcher dialog (maintained by Settings team)

   * IlluminationDrawable.kt

   * LightSourceDrawable.kt

      * These create the glow animation when you tap on a button (see [`qs_media_light_source`](/packages/SystemUI/res/drawable/qs_media_light_source.xml)). Should be reusable in other layouts.

   * Carousel:

      * MediaCarouselController.kt

         * Keeps the carousel view up to date and handles state changes (e.g. expansion)

         * Handles settings gear and page indicator

      * MediaCarouselScrollHandler.kt

         * Handles scrolling between players in the carousel

      * MediaScrollView.kt

         * Scrollview used in the carousel layout, has some custom measurement code

   * Individual players:

      * KeyguardMediaController.kt

         * Lockscreen media controls have a special wrapper in order to work with the existing lockscreen notification layout

      * MediaControlPanel.java

         * Main class for media control UI

      * SeekBarObserver.kt

         * Updates seekbar state

      * SeekBarViewModel.kt

         * Implements its own `computePosition()` for the seekbar (to avoid continually polling the `PlaybackState`, which involves binder calls)

         * Does some touch falsing (ignore flings, require drags to start near the thumb - otherwise users would often accidentally trigger the seekbar when they meant to move the carousel or shade)

      * PlayerViewHolder.kt

         * Holds references to the UI elements in the panel

* Animation support:

   * MediaHierarchyManager.kt

      * Responsible for placement of media view and animation between hosts

   * MediaHost.kt

      * Every location that a media player could be located needs a `MediaHost`

      * Tracks configuration (if it should show inactive media, needs falsing, etc.)

   * MediaHostStatesManager.kt

      * Manages the various media host states and coordinates heights between different players

      * Has the most up to date state for any location

   * MediaViewController.kt

      * Controls a single instance of a media player, keeps the media view states up to date

* Backend

   * MediaData.kt

      * Holds all the media data (track info, active/resume state, etc.)

   * MediaDataCombineLatest.kt

      * Combines update events from `MediaDataManager` and `MediaDeviceManager`, so that downstream listeners will have device info

   * MediaDataFilter.kt

      * Filters media data based on the current user

      * Exit point for the pipeline: "external listeners" (currently `MediaHost` and `MediaCarouselController`), while they should be added via `MediaDataManager.addListener()`, will actually be listening to this output

   * MediaDataManager.kt

      * Entry point for the pipeline; initializes listener connections and assigns external listeners to the correct exit point

      * Converts media notifications and resumable media info into `MediaData`

   * MediaDeviceManager.kt

      * Handles device updates

   * MediaFeatureFlag.kt

      * Utility to check whether media controls are enabled

   * MediaSessionBasedFilter.kt

      * Filters media events based on media session. This prevents duplicate controls in situations like casting where we might get both a local and remote object for the same media session.

   * MediaTimeoutListener.kt

      * Listens to `PlaybackState` and marks controls inactive after the media has been paused/stopped for 10 minutes (value can be adjusted locally with `adb shell setprop debug.sysui.media_timeout [ms]`)

   * MediaResumeListener.kt

      * Listens for new media data and attempts to find a valid `MediaBrowserService` for the app. If successful, sends the information back to the `MediaDataManager`

      * Saves up to 5 valid `MediaBrowserService` components found this way, and queries them for recent media on boot or user change

      * Note: the user can disable this feature completely (or block certain apps from being resumable) in [Settings](https://source.corp.google.com/android/packages/apps/Settings/src/com/android/settings/sound/ResumableMediaAppsController.java), in which case this listener will do nothing (or ignore updates from the blocked apps).

   * ResumeMediaBrowser.java

      * Connects to an app's [`MediaBrowser`](https://developer.android.com/reference/android/media/browse/MediaBrowser) to determine whether SystemUI is able to connect and find a recent [`MediaItem`](https://developer.android.com/reference/android/media/browse/MediaBrowser.MediaItem)

* Factory classes (for unit testing):

   * LocalMediaManagerFactory.kt

   * MediaBrowserFactory.java

   * MediaControllerFactory.java

   * ResumeMediaBrowserFactory.java

## Miscellaneous

Other useful documents:

* [go/sysui-media-resumption-requirements](https://goto.google.com/sysui-media-resumption-requirements) - Internal documentation for app developers about how to work with media resumption

* [Playing nicely with media controls](https://android-developers.googleblog.com/2020/08/playing-nicely-with-media-controls.html) - blog post on the Android 11 updates

* [Media Controls developer guide](https://developer.android.com/guide/topics/media/media-controls)