libsnapshot: remove snapshots properly after flashing

If updated then immediately flashed target slot at unverified
stage, update_engine attempts to call RemoveAllSnapshots, but
it used to fail because unmapping fails. In reality, these devices
are dm-linear targets, so unmapping them is optional.

- Introduce a GetSnapshotFlashingStatus function that expose more
information than AreAllSnapshotsCancelled. We can use the returned
map<string, bool> to determine whether a partition is flashed or
not.
- Introduce ShouldUnmapDeleteSnapshot which determines whether
unmapping / deleteting a snapshot is needed in RemoveAllSnapshots.

Test: apply OTA, flash target slot, then inspect logs
Bug: 147696014
Change-Id: I0853d1e213566af2a3401e46fac7d9586cee7aaf
Merged-In: I0853d1e213566af2a3401e46fac7d9586cee7aaf