keyGesture: set up WearInputService

- Set up flag for wear key gesture handling migration.
- Start WearInputService in SystemServer.
WearInputService will be a system service that centralizes the input
related logic for WearOS.

Bug: 368159846
Bug: 439077699
Flag: com.android.server.policy.wear_key_gesture_handling
Test: manual
Change-Id: I7b3ffadeec35248418ad86ecbe3abddb1c0d6989