a11y: Refactory key gesture dialog delegate

This CL is to refactory KeyGestureDialogStartable. Add
DialogBehaviorDelegate. Factor distinct dialog logic into separate
implementations of DialogBehaviorDelegate.

Bug: b/441151362
Flag:com.android.hardware.input.enable_talkback_and_magnifier_key_gestures
Test: EXEMPT for refactory

Change-Id: I9eb61d5332a04f87bff7f846784bc588bbfcd285