[Media ML] Support customization for volume keys

Create KeyEventHandler class that is extended separately for media
and volume KeyEvents. The logic for long press and
single/double/triple taps will be the same for both KeyEvent types.

Bug: 157988289
Test: manually
Change-Id: I0df16c99d0f07714845c3324d24a804c2923ad28
(cherry picked from commit 4d7d6d83)