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

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



Change-Id: I9c61dc312c22291e928befa115ab596d2d19d80f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>