Log metrics for RKP fallback event.

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

Test: atest com.android.remoteprovisioner.unittest.ServerToSystemTest#testFallback
on cuttlefish, followed by ./statsd_testdrive 10124
Bug: 194510784
Change-Id: I3d6184df02828fef1d0110688afe1ba2db10eae9
(cherry picked from commit 400b225c94329c5192695c15bd2f81568fc92f5e)