Bug: 5011907
Introduce a 150ms delay in handling volume down keys
while waiting to see if a power key will follow.
Don't trigger the screenshot chord if both volume up and
volume down are pressed together.
Don't trigger the long-press power menu if volume keys are
also pressed.
Require the user to press both keys in the chord within
the debounce time and continue long-pressing them in order
to trigger the screenshot action.
Change-Id: I248968d37b73c09d6d08e7f62667c443eba32da0
d5bb82d18c