libsnapshot: Resume snapshot merge if snapshots are in second

phase

If the device reboots when SnapshotUpdateStatus switches from
first phase to second phase, then track the transition
and resume the merge.

Bug: 374225913
Test: OTA on Pixel - Verify merge resumes when device reboots just after
first phase merge
Change-Id: I5f62a03852a4b012850b11d0c1e6b96ec0556278
Signed-off-by: Akilesh Kailash <akailash@google.com>