Revert Media 1.0 API changes introduced for making 1.0 updatable

This CL reverts API changes which were introduced by below CLs:
 - "761c4759: AML: Make config_mediaMetadataBitmapMaxSize as @SystemApi"
 - "888b791f: Make AudioManager.FLAG_FROM_KEY as @SystemApi"
 - "3d1353aa: AML: Unhide KeyEvent.isMediaSessionKey()"

Bug: 128245348
Test: atest CtsMediaTestCases:android.media.cts.MediaSessionTest;
      atest CtsMediaTestCases:android.media.cts.MediaControllerTest;
      atest CtsMediaTestCases:android.media.cts.MediaBrowserTest;
Change-Id: I263a958bf340336f699adb28424241fd00e46e28