libsnapshot: Also use empty space in super for COW

The super partition usually has some empty space even after
the target partitions are created, especially for retrofit
Virtual A/B devices. Use that empty space for COW before taking
up userdata space.

- PartitionCowCreator computes free regions in super partition metadata
  and use that space until it is used up. It returns a pair of numbers
  (cow_partition_size, cow_file_size) and let SnapshotManager to create
  the partition / images with proper sizes.
  - A region is considered free iff it is used by NEITHER target NOR
    source partitions
  - The list is in PartitionCowCreator's return value so that
    SnapshotManager can use it as a guide when creating partitions.
  - These partitions are created under the group named "cow".
    - Avoid mapping COW partitions directly during init first stage
      mount. Init only maps them when they are needed by the top-level
      device.
- CreateCowImage no longer zero-fills the first 4 bytes of the image.
  (See below)
- CreateUpdatePartitions: after creating the snapshot, also maps the COW
  devices (via MapCowDevices()) and zero-fills the first 4 bytes of the
  top-level COW device.
- Add a new SnapshotManager::MapCowDevices() function, which maps both
  the COW partition (in super) and the COW image (through
  IImageManager) (if necessary). Then, create a dm-linear target that
  concatenates them (if necessary).
- Add a new SnapshotManager::UnmapCowDevices() functions that does the
  reverse MapCowDevices().
- DeleteSnapshot also unmaps the top-level COW device and COW partition
  before unmapping the COW images (before deleting them).

Test: libsnapshot_test

Change-Id: I0098b7e842ab48b0b4dd2a59142098b098d23d34