[A11y][Keyboard shortcuts] Add flag for TalkBack key gesture

* This adds a flag to enable just the TalkBack key gesture shortcut
  individually.
* This separates out existing logic that tied together the key gestures
  for TalkBack and Magnification.
* To avoid rolling back existing work for Magnification, the
  enable_talkback_and_magnifier_key_gestures flag will be left named
  as-is.
* Due to the necessity of showing the permissions warning dialog when
  the TalkBack shortcut is added, the technical limitations on the
  existing dialog that shows when TalkBack settings are enabled, and the
  level of UI, behavior, and string change involved in enabling this
  scenario, it is safest to enable this TalkBack key gesture separately.

Bug: 331682600
Bug: 439624730
Flag: com.android.hardware.input.enable_talkback_and_magnifier_key_gestures
Flag: com.android.hardware.input.enable_talkback_key_gestures
Test: Tested manually locally, updated AccessibilityManagerServiceTest
and KeyGestureControllerTests

Change-Id: I7f78e3bffdab1da10a8b4226bc76824bdb4205c4