libsnapshot: Fix tests that depend on PrepareOneSnapshot().

PrepareOneSnapshot was hardcoded in a way that only worked with
pre-compression devices. This patch makes it use the public API and
supported update flow.

One test, SnapshotTest.Merge, now uses OpenSnapshotWriter instead of
MapUpdateSnapshot. There are still other tests using the old API call.

Bug: N/A
Test: vts_libsnapshot_test
Change-Id: Iec4bf6efe6a82e1f90b81fa4211201845ebabe62