Log metrics for RKP fallback event.

This CL adds metrics logging for fallback during RKP hybrid mode.

Ignore-AOSP-First: No merge path from AOSP.
Test: atest com.android.remoteprovisioner.unittest.ServerToSystemTest#testFallback
on cuttlefish, followed by ./statsd_testdrive 10124
Bug: 194510784
Merged-In: I3d6184df02828fef1d0110688afe1ba2db10eae9

Change-Id: I3d6184df02828fef1d0110688afe1ba2db10eae9
1 file changed