Add initial API for stylus and mouse buttons.

Added the concept of pointer properties in a MotionEvent.
This is currently used to track the pointer tool type to enable
applications to distinguish finger touches from a stylus.

Button states are also reported to application as part of touch events.

There are no new actions for detecting changes in button states.
The application should instead query the button state from the
MotionEvent and take appropriate action as needed.

A good time to check the button state is on ACTION_DOWN.

As a side-effect, applications that do not support multiple buttons
will treat primary, secondary and tertiary buttons identically
for all touch events.

The back button on the mouse is mapped to KEYCODE_BACK
and the forward button is mapped to KEYCODE_FORWARD.

Added basic plumbing for the secondary mouse button to invoke
the context menu, particularly in lists.

Added clamp and split methods on MotionEvent to take care of
common filtering operations so we don't have them scattered
in multiple places across the framework.

Bug: 4260011
Change-Id: Ie992b4d4e00c8f2e76b961da0a902145b27f6d83