MediaSourceImpl: Avoid race condition when linking with agnostic-added signal

Change-Id: I39402955884479f76c56c04245d16f9834dd0773
MediaSinkImpl: Return false if sink pad cannot be created

Change-Id: I514bad890f59ff8196dbb55f8a0f7bcb418e1b2d
agnosticbin_test: Fix possible race condition using g_atomic badly

Change-Id: Id2de6df8c09daff561feccd3714e9184d37b8261