Merge changes from topic "libsnapshot_api_update_engine"

* changes:
  libsnapshot: tests for public APIs.
  fastbootd: skip COW group
  libsnapshot: no overlayfs during virtual a/b ota.
  libsnapshot: Also use empty space in super for COW
  libsnapshot: APIs for all partitions
  fs_mgr: CreateDmTable takes CreateLogicalPartitionParams