MountService: Add support for multiple storage configurations

Currently AOSP only supports a single storage configuration that is hardcoded
in storage_list.xml. Some device updates are now merging data and internal
storage partitions, and as a result require the storage list to support two
different variations.

By utilizing the sys.storage_legacy property this will allow for the framework
to support multiple configurations which can be determined during boot.

Change-Id: Id795520425ea3e99a5234e10c978dcdbd68a1311