Allow reliable detection of a message that is in use.

Because the standard Looper.loop code calls Message#recycle it is
imperative that Handler#handleMessage code not attempt to resue
a message it receives. If allowed to do so it will cause bugs that
could be difficult to diagnois.

This change adds Message#flags and uses one bit to reliably detect
a message is in use and throws an error in MessageQueue#enqueueMessage.
This allows early detection of this bug.

Note: This is not new functionality, but the current implementation does
not detect messages that are in use because it uses Message#when != 0
as the detection mechanism. The problem is that a Message#when value of 0
is valid value used to place a message at the front of the queue and is
thus unreliable.

Another option is to change the setting of Message#when in Message#enqueueMessage
so that it is never 0, although that does change subtly a publicly accessible
field.

Yet another option would be to use other fields but all candidates have
similar problems as when in that they are publicly accessible or even
settable such as Message#target.

Change-Id: I2df600537700a3fe206678f38bcae7329751c4e5