surfaceflinger: distinguish mCurrentParent/mDrawingParent

Updates to wp<> is not atomic.  We cannot use/update it at the same
time from the main thread and a binder thread.  With this change,
binder threads use mCurrentParent with the external state lock held.
The main thread uses mDrawingParent.

This is also an alternative fix to bug 62099658 and allows us to
revert "SurfaceFlinger: Update parent pointer while performing
transaction."

Bug: 38505866
Bug: 62099658
Test: boots and no repro (but I can never repro)
Change-Id: Id286a437537daaeec5eee5de62b1d9df245ece53