Add public getMediaKeyEventSession(PackageName)

The MediaSessionManager has two system APIs
 - getMediaKeyEventSession
 - getMediaKeyEventSessionPackageName
which get the current the media key session and its package name.

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: I18904fb68ad175a668f79deb77ced507b7fc800b