SurfaceFlinger: Emit TransactionCompleted callbacks directly.

Transaction complete callbacks (BLAST Buffer release callbacks), are
emitted from a TransactionCompletedThread. There's no clear reason
for this behavior, as the actual runtime is exceedingly minimal. This
opens us to scheduling delays when we call
TransactionCompletedThread.sendCallbacks. In this CL we both remove the
threaded nature of TransactionCompletedThread, and also take advantage
of it running on the main thread to reduce some unnecessary copying and
further reduce its runtime.

Bug: 176691195
Test: Existing tests pass
Change-Id: Ib1661df24c4a2ee39fc28c29a395158ce0ee7b7f