libsnapshot: CreateUpdateSnapshots reuse COW space

At the second update for Virtual A/B device,
PartitionCowCreator thinks the COW partitions in
the existing super metadata as occupied. In fact, these
partitions aren't used after the merge.

Now, unmap these partitions from the device mapper. If
something bad happens and the previous update has not been
merged yet, the unmap will fail. Then, delete these old
COW partitions from the device.

Test: Virtual A/B update twice
Bug: 135752105
Change-Id: Iab867ded19755089e6e0800e553a10fbcddbb931