SurfaceFlinger: add more thread-safety annotations.

As part of debugging a crash in SurfaceFlinger, we became suspicious of
markLayerPendingRemovalLocked because it wasn't annotated with thread
safety annotations. This ended up being a red herring, because
mStateLock is held on all paths into that function. Add some helpers to
MutexUtils.h to require/assert mutexes, and use them to sprinkle some
thread safety annotations to make things more clear.

Test: m surfaceflinger
Change-Id: Ib48433765bb6ffcb351ec10cd5bc89e254b48545
Merged-In: Ib48433765bb6ffcb351ec10cd5bc89e254b48545