Limit interactions to only the current pad

Bug: 18908042

- Hide the descendents of the non-current pad using the
  IMPORTANT_FOR_ACCESSIBILITY_NO_HIDE_DESCENDANTS flag.
- Intercept touch events that may be sent to the non-current pad.
- Allow DPAD key events to be handled by the formula TextView so that
  it can be scrolled using a hardware keyboard.

Change-Id: Ib92c72f939a6b7469eed060adcc5cefdcfc98666