libsnapshot: Refactor: add {Create,Map,Unmap}CowImage

Move operations on image manager in *Snapshot functions to their own
functions for finer granularity in control. *Snapshot functions only
changes snapshot state and snapshot devices, but not the supporting
devices.

Now, MapSnapshot can take a customized COW device path. We will have
a more complicated stack for the COW device in upcomming CLs.

Also, Change SnapshotManager::CreateSnapshot's signature to
accept a SnapshotStatus struct that includes all sizes, so that
cow_partition_size and cow_file_size can also be written to the snapshot
status file.

Test: libsnapshot_test

Change-Id: I388ecd4bcfbfcc3f379ecb6993615234f4fbcb4e