Queues, queues, queues and input.

Redesigned how ViewRootImpl delivers input events to views,
the IME and to native activities to fix several issues.

The prior change to make IME input event delegation use
InputChannels failed to take into account that InputMethodManager
is a singleton attached to the main looper whereas UI may be
attached to any looper.  Consequently interactions with the
InputChannel might occur on the wrong thread.  Fixed this
problem by checking the current thread and posting input
events or callbacks to the correct looper when necessary.

NativeActivity has also been broken for a while because the
default event handling logic for joysticks and touch navigation
was unable to dispatch events back into the native activity.
In particular, this meant that DPad synthesis from touch navigation
would not work in any native activity.  The plan is to fix
this problem by passing all events through ViewRootImpl as usual
then forwarding them to native activity as needed.  This should
greatly simplify IME pre-dispatch and system key handling
and make everything more robust overall.

Fixed issues related to when input events are synthesized.
In particular, added a more robust mechanism to ensure that
synthetic events are canceled appropriately when we discover
that events are no longer being resynthesized (because the
application or IME is handling or dropping them).

The new design is structured as a pipeline with a chain of
responsibility consisting of InputStage objects.  Each InputStage
is responsible for some part of handling each input event
such as dispatching to the view hierarchy or to the IME.
As a stage processes an input event, it has the option of
finishing the event, forwarding the event to the next stage
or handling the event asynchronously.  Some queueing logic
takes care to ensure that events are forwarded downstream in
the correct order even if they are handled out of order
by a given stage.

Cleaned up the InputMethodManager singleton initialization logic
to make it clearer that it must be attached to the main looper.
We don't actually need to pass this looper around.

Deleted the LatencyTimer class since no one uses it and we have
better ways of measuring latency these days using systrace.

Added a hidden helper to Looper to determine whether the current
thread is the indicated Looper thread.

Note: NativeActivity's IME dispatch is broken by this patch.
This will be fixed later in another patch.

Bug: 8473020
Change-Id: Iac2a1277545195a7a0137bbbdf04514c29165c60