Log staged rollback result after reboot

After rebooting a device to apply a staged rollback, we log
with statsd the result of that operation.

Test: manual:
With local modifications to the CL to debug log when stats are logged
and to make the rollback test app the module metadata app:
* adb install RollbackTestAppAv1.apk
* adb install --staged --enable-rollback RollbackTestAppACrashingV2.apk
* Launch RollbackTestApp repeatedly until device reboots.
* Device should have rebooted and test app been rolled back.
* Confirm logs for ROLLBACK INITIATE, BOOT_TRIGGERED, and SUCCESS

Bug: 120598832
Bug: 126887433
Change-Id: Ia05a506592ec474609334e931d5295c7a3166684