Adding APIs for an accessibility service to intercept key events.

Now that we have gestures which are detected by the system and
interpreted by an accessibility service, there is an inconsistent
behavior between using the gestures and the keyboard. Some devices
have both. Therefore, an accessibility service should be able to
interpret keys in addition to gestures to provide consistent user
experience. Now an accessibility service can expose shortcuts for
each gestural action.

This change adds APIs for an accessibility service to observe and
intercept at will key events before they are dispatched to the
rest of the system. The service can return true or false from its
onKeyEvent to either consume the event or to let it be delivered
to the rest of the system. However, the service will *not* be
able to inject key events or modify the observed ones.

Previous ideas of allowing the service to say it "tracks" the event
so the latter is not delivered to the system until a subsequent
event is either "handled" or "not handled" will not work. If the
service tracks a key but no other key is pressed essentially this
key is not delivered to the app and at potentially much later point
this stashed event will be delivered in maybe a completely different
context.The correct way of implementing shortcuts is a combination
of modifier keys plus some other key/key sequence. Key events already
contain information about which modifier keys are down as well as
the service can track them as well.

bug:8088812

Change-Id: I81ba9a7de9f19ca6662661f27fdc852323e38c00