Merge "audio: Update README.md to add info about the AIDL version"

per-file README.md = elaurent@google.com,mnaganov@google.com

Directory structure of the audio HAL related code.

Run `common/all-versions/copyHAL.sh` to create a new version of the audio HAL

based on an existing one.

## Directory Structure for AIDL audio HAL

## Directory Structure

The AIDL version is located inside `aidl` directory. The tree below explains

the role of each subdirectory:

* `aidl_api` — snapshots of the API created each Android release. Every

  release, the current version of the API becomes "frozen" and gets assigned

  the next version number. If the API needs further modifications, they are

  made on the "current" version. After making modifications, run

  `m <package name>-update-api` to update the snapshot of the "current"

  version.

* `android/hardware/audio/common` — data structures and interfaces shared

  between various HALs: BT HAL, core and effects audio HALs.

* `android/hardware/audio/core` — data structures and interfaces of the

  core audio HAL.

* `default` — the default, reference implementation of the audio HAL service.

* `vts` — VTS tests for the AIDL HAL.

## Directory Structure for HIDL audio HAL

Run `common/all-versions/copyHAL.sh` to create a new version of the HIDL audio

HAL based on an existing one. Note that this isn't possible since Android T

release. Android U and above uses AIDL audio HAL.

* `2.0` — version 2.0 of the core HIDL API. Note that `.hal` files

  can not be moved into the `core` directory because that would change