SDK path: frameworks/base/core/java/android/bluetooth/
Service path: packages/apps/Bluetooth/src/com/android/bluetooth/
This class cannot be found when using the protocol class, because the Client part or related interfaces of the Bluetooth protocol in the Android source code are hidden by @hide, so android.jar cannot meet the needs of Android source code framework layer developers. Reflection mechanism or reference framework.jar instead of android.jar.
位置：out\target\common\obj\JAVA_LIBRARIES\framework_intermediates\classes.jar
1. Introduction to the protocol version
The AVRCP protocol version changes, and the versions are all downward compatible:
v1.0 : basic remote control commands, such as play, pause, cut song
v1.3 : Added the ability to obtain the current playing status of the music and the song information of the playing music (total duration of the song, current playing position, song name, album name, singer)
v1.4 : Add browsing function, support absolute volume adjustment
v1.5 : Changes passed to the related protocol to correct various errors
v1.6 : Added two new features:
1) Number of items: interface for controller, request and receive number of items in folder without downloading list
2) Cover Art: Supports transferring images to media items via the BIP (Basic Imaging Profile) protocol based on the OBEX protocol