Merge "[Media TTT] Add more information about code architecture to the...

# Media Tap-To-Transfer

## Overview

This package (and child packages) include code for the media tap-to-transfer feature, which

allows users to easily transfer playing media between devices.

In media transfer, there are two devices: the *sender* and the *receiver*. The sender device will

start and stop media casts to the receiver device. On both devices, System UI will display a chip

informing the user about the media cast occurring.

In media transfer, there are two devices: the **sender** and the **receiver**. The sender device

will start and stop media casts to the receiver device. On both devices, System UI will display a

chip informing the user about the media cast occurring.

This package is structured so that the sender code is in the sender package, the receiver code is

in the receiver package, and code that's shared between them is in the common package.

**Important**: System UI is **not responsible** for performing the media transfer. System UI

**only** displays an informational chip; external clients are responsible for performing the media

transfer and informing System UI about the transfer status.

## Information flow

External clients notify System UI about the transfer status by calling `@SystemApi`s in

`StatusBarManager`. For the sender device, use the `updateMediaTapToTransferSenderDisplay` API; for

the receiver, use the `updateMediaTapToTransferReceiverDisplay` API. The APIs eventually flow into

SystemUI's `CommandQueue`, which then notifies callbacks about the new state.

`MediaTttChipControllerSender` implements the sender callback, and `MediaTttChipControllerReceiver`

implements the receiver callback. These controllers will then show or hide the tap-to-transfer chip

(depending on what information was sent in the API).

## Architecture

This package is structured so that the sender code is in the `sender` package, the receiver code is

in the `receiver` package, and code that's shared between them is in the `common` package.

* The `ChipStateSender` and `ChipStateReceiver` classes are enums that describe all the possible

  transfer states (transfer started, transfer succeeded, etc.) and include relevant parameters for

  each state.

* The `ChipSenderInfo` and `ChipReceiverInfo` classes are simple data classes that contain all the

  information needed to display a chip. They include the transfer state, information about the media

  being transferred, etc.

* The `MediaTttChipControllerSender` and `MediaTttChipControllerReceiver` classes are responsible

  for showing or hiding the chip and updating the chip view based on information from the

  `ChipInfo`. `MediaTttChipControllerCommon` has all the common logic for adding and removing the

  view to the window and also includes any display logic that can be shared between the sender and

  receiver. The sender and receiver controller subclasses have the display logic that's specific to

  just the sender or just the receiver.

## Testing

If you want to test out the tap-to-transfer chip without using the `@SystemApi`s, you can use adb

commands instead. Refer to `MediaTttCommandLineHelper` for information about adb commands.