Merge "Changing BluetoothHeadset APIs from @hide to @SystemApi" into stage-aosp-master am: 913ea0a167 am: 41dc0fb942 am: 712c9b7c3a

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/15728282

Change-Id: I7c88dd1c79704c8f97f1112d313244da6d74ed20