[Media ML] Unhide dispatch key event APIs

When dispatching key events, PhoneWindowManager and AudioManager uses
MediaSessionLegacyHelper, which in turn uses MediaSessionManager.
This CL exposes the MediaSessionManager APIs as system API since
MediaSessionLegacyHelper will not be moved to mainline.

Bug: 176953066
Test: m update-api
Change-Id: Ib7b06f076904ea464c31314279a2ced451f4c46e