Extend atoms with Virtual A/B merge metrics

With the Virtual A/B feature, when the device successfully boots after
an OTA update, a dm-snapshot merge operation is performed, which
consolidates the updated images to the base devices.
This merge operation depends on the OTA size and may affect the device
performance until completed.
The new virtual_ab_update_reported push metric tracks the merge
operation duration (if not interrupted), the final merge status and
possible number of reboots that occurred during the merge.

Bug: 138817833
Bug: 148138124
Test: build + OTA application + local metric test with statsd_testdrive
Change-Id: Iabd1dc48ad0d737f6ed14caa247fd4952c65a258
Signed-off-by: Alessio Balsini <balsini@google.com>
Merged-In: Iabd1dc48ad0d737f6ed14caa247fd4952c65a258