CheckBox should not populate AccessibilityEvent with text.

1. Accessibility services are the ones that choose how to
   announces the checked state of a checkable control, so
   CheckBox should not add strings for its state to access
   events.

2. Removed some unused accessibility related strings.

bug:6241115

Change-Id: I572b961191da4b3537fb6cad529d9764d39161ec