init: Add first-stage init support for snapshot-based partitions.

Note that /metadata must now be mounted before CreateLogicalPartitions()
is called. This is because SnapshotManager overrides the default
partitioning scheme, and the only way to tell if a SnapshotManager is
needed is via the metadata partition.

Bug: 139204329
Test: manual test
Change-Id: I812df6c4c0d4d8753b1516f63dc70c5bc3e1c09c
3 files changed