a11y: Ignore other dialogs while there is a dialog showing

This CL will update the logic to that if there is a dialog showing, when
the user presses confirmation dialog shortcuts, we will still show the
existing dialog.

Bug: b/436349085
Flag:com.android.hardware.input.enable_talkback_and_magnifier_key_gestures
Test: atest KeyGestureDialogStartableTest & manual
Change-Id: Icebe65d1798cd980b0885628101cf1aeb671c740