Use FLAG_LONG_PRESS for headset long press interactions.

Handle canceled key events correctly and don't synthesize
key events in that case.

Unfortunately, the state machine was confused by some sequences
of key events that it might receive from the input dispatcher
when new activities take focus during a long-press on the headset key.
The audio service may receive a cancel event intended for the old
window, followed by a repeated down and finally an up for the new window.
Simplified this down to just two booleans.

Bug: 6484717
Change-Id: I9587d0a5e282419ef4d7c17665940682aacea96a