Make Message.setAsynchronous() public.

There are many cases in real world applications where it is desirable
to continue processing messages on the Looper even when most other
messages have been suspended by a synchronization barrier pending
completion of the next drawing frame on vsync.

Internally the framework is able to mark certain messages as being
independent of these higher level synchronization invariants by
flagging them as asynchronous.

This change exposes the existing function and improves on the
documentation so that it is clearer what is meant by asynchronous.

Bug: 18283959
Change-Id: I775e4c95938123a364b21a9f2c39019bf37e1afd