Check if already on main thread when getting SSG for SCVH

SCVH can be added in process so if they are included in a SSG
the call to getSurfaceSyncGroup could run on the main thread.
If this were the case, the method would never return a value
since the call is is blocking and waiting to run on the main
thread. Instead, check if the incoming thread is the main thread
and return a value immediately instead of a post to main thread.

Change-Id: Id233eecc46afb1f5425df97986e7563e970aff12
Bug: 276356641
Test: SurfaceSyncGroupContinuousTest