Merge "Make BluetoothHeadsetClient and associated functions @SystemApi" am: e70578157d am: 881867d5e1 am: ab0e7cd2cf

Original change: https://android-review.googlesource.com/c/platform/frameworks/base/+/1954516

Change-Id: I5f76c8473b66c345410306eb0b32f6491cde0257