snapshotctl: send snapshot merge report metrics

Add the --report flag to spshotctl to submit Android metrics on the final state
of the snapshot merge operation, the duration if the merge is completed in a
single session, or the number of reboots that happened before completing the
merge.

Bug: 138817833
Bug: 148138124
Test: m + OTA + statsd_testdrive
Change-Id: Id657f33963bb9926553fa6eeb1721a5229f11e87
Signed-off-by: Alessio Balsini <balsini@google.com>
Merged-In: Id657f33963bb9926553fa6eeb1721a5229f11e87