Added accessibility category to shortcut helper

Added new category to shortcut helper as ground work to add new A11y
shortcuts to shortcut helper

Test: None - there are no accessibility shortcuts added yet so they will
not show up on shortcut helper
Flag: com.android.hardware.input.keyboard_a11y_shortcut_control
Fix: 382016793

Change-Id: Ia6a52a518c9382126e2f3e522a9d37f8139b4414