libsnapshot: Pass the correct source device to snapuserd.

When a partition shrinks, it is not correct to use the base device as
the "source" device for the new COW format, because we may need to read
blocks that do not exist in the new partition.

To resolve this, we store a copy of the old partition layout in /metadata,
and use it to create a "source" view of the old partition. The new
stacking looks as follows:

  partition_b (dm-snapshot):
   - partition_b-base (partition_b dm-linear)
   - partition_b-cow-user (dm-user + snapuserd):
      - partition_b-cow (COW image)
      - partition_b-src (partition_a dm-linear)

Bug: 177935716
Test: vts_libsnapshot_test
Change-Id: I872f271cc1f25cc796b94188fdde247cdc4050b4