Ensure that framework does not refer MediaSession2 related code

Also, this CL moves mediasession2 to com.android.media apex.

Bug: 123495968
Test: Passed below tests:
atest CtsMediaTestCases:android.media.cts.MediaSession2Test;
atest CtsMediaTestCases:android.media.cts.MediaController2Test;
atest CtsMediaTestCases:android.media.cts.MediaItem2Test;
atest CtsMediaTestCases:android.media.cts.MediaSession2ServiceTest;
atest CtsMediaTestCases:android.media.cts.Session2CommandGroupTest;
atest CtsMediaTestCases:android.media.cts.Session2CommandTest;
Change-Id: I8cf7a657545e6b7b2973035f365506206e7147a0