TalkBack and switch access support for CropView

- Switch to androidx ExploreByTouchHelper, add View pass-through calls.
- Add virtual view IDs for left/right bounds as well.
- Make content description describe the position (as a %)
- Allow volume buttons to move the current boundary along its axis
  (matching pattern used in Photos)
- Remove unused click action for boundaries (was breaking switch access)

Bug: 184018281
Bug: 184012364
Bug: 180062481
Bug: 184105678

Test: Enable talkback, move crop bounds with volume keys, same with
      switch access
Change-Id: I06bab02a37620f19e15cddcfc79ae858a1614e92