Import wear stem primary button gesture to master.

Features imported in this change:
1. Single press stem primary button to open launcher.
2. Double press stem primary button to switch to most recent task.
3. Triple press stem primary button to toggle accessibility mode.
4. Long press stem primary button to trigger assistant.

Bug: 197273843
Test: manual test

Change-Id: I0e0d11b042f8ae966efb98f5bd4e25f906292330