libsnapshot: Add helper for first-stage init mounting

With this patch, init can mount snapshots in the first stage by
performing the following operations:
 1. First, check if SnapshotManager::kBootIndicatorPath exists.
 2. If so, call SnapshotManager::NewForFirstStageMount.
 3. If NeedSnapshotsInFirstStageMount returns true,
 4. Call CreateLogicalAndSnapshotPartitions().

When called, this replaces any calls to CreateLogicalPartitions().

Rather than split this into multiple functions (to generate uevents as
needed), we instead use major:minor strings for device-mapper tables.
This means we don't have to wait for paths to resolve.

Bug: 139204329
Test: libsnapshot_test gtest
Change-Id: Ia7ec196a62e51748d6f01a66fe4e9eef25f2898f