Add new flag to guard refactoring for multi key gestures

Part 2 of the key gesture refactoring:
- KeyCombinationManager gestures
- SingleKeyGestureDetector gestures
- Home key gestures

Bug: 358569822
Test: None
Flag: com.android.hardware.input.use_key_gesture_event_handler_multi_press_gestures
Change-Id: I5828f729c20fc2d4a11a5fe77f140d9f15d56757