Flush BC_FREE_BUFFER and ref ops from non-looper threads.

BC_FREE_BUFFER and ref commands are normally just queued, and not
automatically flushed out to the kernel driver. This usually works fine,
because BC_FREE_BUFFER is typically called from a binder thread (which
flushes when calling back into the kernel), or a thread making regular
binder transactions itself.

But it can happen that a Parcel is destructed from a thread that meets
neither of those requirements; especially Parcels created from Java are
sensitive to this, because if they aren't immediately recycled, they
will instead be garbage collected, and in that case the BC_FREE_BUFFER
will be queued to the FinalizerDaemon thread, which otherwise never
makes or receives any binder calls.

To prevent these commands from getting stuck, flush BC_FREE_BUFFER and
refcount operations automatically from such threads.

Bug: 68604253
Bug: 139697085
Test: boots, binderLibTest
Change-Id: I98109a7046c122db22af0b15a268629284f06663