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: 213031779
Bug: 213253413
Bug: 193549218
Ignore-AOSP-First: cherry-pick from AOSP
Test: inject transient failure into CheckMergeConsistency, apply OTA,
      reboot and complete merge.
Change-Id: I31fdae6bde48e3a71b6f3fcc663541257f7ebd8f
Merged-In: I31fdae6bde48e3a71b6f3fcc663541257f7ebd8f