Add relative coordinates for touchpad events.

By their nature, touchpad devices deal in absolute coordinates. It
may prove useful for app developers to receive information about how
touches have changed position since the last event.

Test: connect a device that android classifies as a touchpad (such
      as a DualShock 4 in touchpad capture mode). MotionEvents
      received in an app should have valid content in AXIS_RELATIVE_X
      and AXIS_RELATIVE_Y fields.

Bug: 38511270
Change-Id: Ic96323464af6f11027fbf36ee00e5d226c1b8626
Merged-In: Ic96323464af6f11027fbf36ee00e5d226c1b8626