SurfaceFlinger: Only block calling thread on sync transactions.

We use synchronous transactions and synchronous input commands so that
the caller can block until after the transaction has executed. However
the current implementation blocks all other callers. This isn't required
and could create performance issues or binder exhaustion issues so
this CL modifies the code so only the calling thread will block.

Bug: 154443842
Test: Existing tests pass
Change-Id: I556497b1a48de5add753f4de09227b16ca1fc428