DO NOT MERGE. Improve screenshot chord debouncing.

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