avrcp: Extract media control wrappers and helpers

These should be in a separate package with public classes and
methods so that it could be used by both, the AVRCP and new upcoming
Bluetooth media control service.

Tag: #feature
Test: atest MediaPlayerListTest BrowserPlayerWrapperTest MediaPlayerWrapperTest
Bug: 150670922
Change-Id: I95c7d9966c4f348feb6ceea4a8c5fcdb6dbe3f33