Merge "Clarify documentation for why an app might use a PlaybackSession, and...

Merge "Clarify documentation for why an app might use a PlaybackSession, and how to create and use it." into main am: 9fbaf5f7 am: fc99cd5a

Original change: https://android-review.googlesource.com/c/platform/frameworks/base/+/2928590



Change-Id: I4448cb56a3e57383b91d8938b6cb63a6e69f9708
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>