Upstream Wear Keyguard changes

1. KeyguardService#dismissKeyguardToLaunch. It will be called if watch is keyguarded and user does button presses to launch Pay or Assistant. Applied only for watches.
2. KeyguardService#onSystemKeyPressed. Passes stem button press to keyguard.
3. Launch android.intent.action.VOICE_ASSIST_RETAIL for retail assistant button press.
4. Allow ACTION_VOICE_ASSIST to be disabled at runtime.

Bug: 204870856
Tests: manual
Change-Id: Ibc7fd2d57d0f01bdfca4ea3054fb833fb01b5471
(cherry picked from commit 848b4576561120c875430f4fd38806142a08a115)