Only force send callbacks if they are registered

Some clients of ASurfaceControl may not set callbacks.  We do not want
to call finalizePendingCallbacks in this case.

Bug: 159231312
Test: build, boot, SurfaceFlinger_test, libsurfaceflinger_unittest
Change-Id: I62dc270302a60274d8d1192d0e35ce3f48721e6c
1 file changed