Utility class to measure snapshot merge duration

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.
Create the SnapshotMergeStats class that takes care of measuring the duration
of a merge operation and manages the merge state file to continue tracking the
merge in the case of an interruption due to a reboot.

Bug: 138817833
Test: m
Change-Id: I9dcf1423716c89d8f9b4a0feb4e75bb7b9d18935
Signed-off-by: Alessio Balsini <balsini@google.com>