libsnapshot: Propagate merge phase across merge failures.

If a merge fails we write a new snapshot status indicating that the merge
failed. If this happens to occur during the second merge phase, we fail
to propagate the phase counter to the new status. This means the merge
is unlikely to make progress and succeed later.

Bug: 193549218
Test: manual test
Change-Id: I31fdae6bde48e3a71b6f3fcc663541257f7ebd8f