Add new MotionEvent actions for button press and release.

Introduce ACTION_BUTTON_PRESS and ACTION_BUTTON_RELEASE as actions to
signal a button press or release. If these actions happen
simulanteously with a DOWN or UP event then they're explicitly
ordered to happen after the DOWN or preceding the UP in order to send
them to the most recently targeted view.

Also, introduce new stylus button constants that differ from the
constants we use for mouse buttons.

Bug: 20704355
Change-Id: Ib960a5004db5429ad2fc8db020704773e2978327