fastboot: Fix snapshot-update merge behavior.

When merging in recovery, the "imminent data wipe" code was used, which
made the assumption the /metadata and /data state would be zapped. This
caused future OTAs to error because the old snapshots were detected.

This CL allows OTAs to proceed even if unexpected snapshots are present.
It also forces the state to "MergeCompleted" after a merge in recovery,
so that the next normal boot can perform cleanup.

Bug: 155339165
Test: fastboot snapshot-update merge, then take another OTA
      vts_libsnapshot_test
Change-Id: Ief6dea3ba76323044e61307272dda320a4494aea