BufferStateLayer: Use mCurrentState for HeadFrameNumber

This is the frameNumber used for deferred transaction signalling.
We need to use this because of cases where we defer a transaction
for a surface to itself. In the BLAST world this may not make a huge amount of sense
(Why not just merge the Buffer transaction with the deferred transaction?)
but this is an important legacy use case, for example moving a window
 at the same time it draws makes use of this kind of technique. So anyway
imagine we have something like this:
Transaction { // containing
    Buffer -> frameNumber = 2
    DeferTransactionUntil -> frameNumber = 2
    Random other stuff
 }
Now imagine getHeadFrameNumber returned mDrawingState.mFrameNumber
(or mCurrentFrameNumber). Prior to doTransaction SurfaceFlinger
will call notifyAvailableFrames, but because we haven't swapped
mCurrentState to mDrawingState yet we will think the sync point
is not ready. So we will return false from applyPendingState and not swap
current state to drawing state. But because we don't swap current state
to drawing state the number will never update and we will be stuck. This way
we can see we need to return the frame number for the buffer we are about
to apply.

Bug: 146598493
Bug: 149251083
Bug: 149315421
Test: Flip the flag. Play.
Change-Id: Ibac46da8ed48bef868328dfe38b106115f09b91f