Fix several accessibility magnification issues.

Clarifying region used for magnification as "magnificationRegion",
both in the public API and in the code. There's been significant
confusion about what "magnfifiedRegion" means. Removing
"availableRegion" from everywhere except where it's required, as
that region was identical to magnified/magnification region.

Trying to shut down magnification was a complex situation where
animations in progress and new magnification requests were tricky to
handle correctly. It was not possible to guarantee that the
magnification callbacks were unregistered consistently. There were
at least two situations that led to phone restarts:
1. If a triple tap was detected between unregistering the callbacks
and shutting down the input filter. In this case the magnification
request would go through.
2. If an animation had just started when magnification was turned
off, so the current magnification was 1.0 but the animator was
about to change it. In this case the callbacks would be unregistered,
and then the animator would start changing the magnification.

This change makes registering and unregistering magnification atomic.
It also makes MagnificationController stick around indefinitely once it
is created, registering and unregistering as needed to support
magnification gestures and services that control magnification. Services
that merely query the status of magnification no longer register for
callbacks.

One part of shutting down is turning off the animation and guaranteeing
that it won't try to make further changes. Adding a flag to
SpecAnimationBridge and a lock in that class so we can guarantee that
nothing happens when we aren't registered for magnification callbacks.

Also reconfiguring all accessibility options when a service stops to
make sure that only the features required by the current configuration
are enabled.

Bug: 27497138
Bug: 27821103
Change-Id: If697cbd34b117d82c8eee1ba7d0254089ee4241d