Update phone call end requirements

Only stop MediaProjection upon a phone call ending when the phone call
was started prior to the MediaProjection.

This way phone calls established during a MediaProjection session remain
active when the call ends.

Bug: 377877538
Test: atest MediaProjectionStopControllerTest
Flag: com.android.media.projection.flags.stop_media_projection_on_call_end
Change-Id: I112ce51add55655c2efdaa3726e546f541bf699a