Add config_mediaContentControlEnablesFullScan

Which signals the availability of full scan for holders of
MEDIA_CONTENT_CONTROL.

Bug: b/352401364
Test: mp droid
Test: Prototyped and tested locally using cl/655116480
Flag: EXEMPT resource only update
Change-Id: I970c455cb8a031cd8c1cbd0b70d1f5de0011547c