[A11y][Keyboard shortcuts] Add flag for Select to Speak key gesture

* This adds a flag to enable just Select to Speak key gesture shortcut
  individually.
* This separates out existing logic that tied together the key gestures
  for TalkBack, Select to Speak, and Magnification.
* Due to technical limitations on the Select to Speak accessibility service,
  it is safest to enable it separately.

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

Change-Id: I785bb620d91a9a6e2778e3bd0ca47f381686255d