libsnapshot: Improve how devices are collapsed after merging.

Currently, we replace snapshot-merge with a linear device wrapping the
base device. This is not efficient. This patch reads LpMetadata for the
underlying partition, and duplicates its table into the snapshot-merge
device. This removes a layer of stacking and also allows removing the
base device.

Note that snapshot_test is growing a bit unwiedly, because it's starting
to implement pieces of libsnapshot that will be filled in later for
update_engine. (MapUpdatePartitions is a good example of this.) When
those pieces land in libsnapshot, snapshot_test will be cleaned up to
remove much of this manual fiddling.

Bug: 139090440
Test: libsnapshot_test gtest
Change-Id: I3872dc51d9e5980803303806f42a5c7e74b0b78a