RESTRICT AUTOMERGE SurfaceFlinger: fix a potential race condition in stealReceiveChannel Add a mutex to prevent a potential race condition. Bug: 232541124 Test: See bug for details Change-Id: Ia338f124c786bf12d6adba10a67b9048fe9c34a5 (cherry picked from commit 0840cdd08f39994e3f8c58eb65f24a8db1dc1173) Merged-In: Ia338f124c786bf12d6adba10a67b9048fe9c34a5