Implement test harness mode

Test Harness Mode is a feature for device farms that want to wipe
their devices after each test run. It stores the ADB keys in the
persistent partition (used for Factory Reset Protection) then performs
a factory reset by broadcasting the MASTER_CLEAR intent.

Upon rebooting, the Setup Wizard is skipped, and a few settings are set:
  * Package Verifier is disabled
  * Stay Awake While Charging is enabled
  * OTA Updates are disabled
  * Auto-Sync for accounts is disabled

Other apps may configure themselves differently in Test Harness Mode by
checking ActivityManager.isRunningInUserTestHarness()

Bug: 80137798
Test: make && fastboot flashall -w
Test: adb shell cmd testharness enable
Change-Id: I91285c056666e36ad0caf778bffc140a0656fcfa