Fix native crash when message queue quits.

Fix a race when quitting the looper's message queue that could
cause the mPtr field to be zeroed out and the native object to
be destroyed while still in use.

This happened due to an optimization that was intended to release
the native looper's file descriptor as soon as the last message
was processed rather than waiting for the finalizer to run.

Bug: 9726217
Change-Id: I695a9a657acfdb3ce65a5737ff20cd11113d15fa