Add public add/removeOnMediaKeyEventSessionChangedListener

The MediaSessionManager has two system APIs
 - addOnMediaKeyEventSessionChangedListener
 - removeOnMediaKeyEventSessionChangedListener
which listen the media key session updates.

Since the APIs require MEDIA_CONTENT_CONTROL permission,
non-system apps could not use the APIs.

This CL adds an overloaded version of those APIs so that apps with
enabled notification listener can use the functionality.

Bug: 187483932
Test: Passed MediaSessionManager(Host)Test
Change-Id: Ic13353983e23f557afaa13eb11a48e48d24d4358