mDirtyRegion is single threaded, but could be accessed from a hwc thread

We now have mInvalidateRegion which holds the region to invalidate, it
can be set from any thread as long as mInvalidateLock is held. We use
fine-grained locking here because mInvalidateRegion can be set from anywhere,
in particular frmo HWC callbacks.

Bug: 5466774
Change-Id: Iafca20aa3f5b25a87755e65bde7b769aa8f997bc