Floss: Implement API to add media player

Wire API to register media player for the qualification purpose.

It is required in the following tests:
* AVRCP/TG/MPS/BV-02-C
* AVRCP/TG/MPS/BV-05-C
* AVRCP/TG/MPS/BV-06-C
* AVRCP/TG/MPS/BV-07-C

Bug: 275024442
Test: run the above tests
Tag: #floss

Change-Id: I1b1153c0bb182fd0ef0bb71a53e9f87a37be155a