Add talkback, magnification and select_to_speak shortcuts

Add the following shortcuts to a11y category in shortcut helper:
 - KEY_GESTURE_TYPE_TOGGLE_TALKBACK
 - KEY_GESTURE_TYPE_TOGGLE_MAGNIFICATION
 - KEY_GESTURE_TYPE_ACTIVATE_SELECT_TO_SPEAK

Bug: 389652455
Test: verified locally
Flag: com.android.hardware.input.enable_talkback_and_magnifier_key_gestures
Change-Id: I8b5851c1575e906a087ae2214c3c84fda0c0e164