StorageManager: Clean up and generalize storage configuration resources

Replace config_emulateExternalStorage, config_externalStorageRemovable,
config_externalStoragePaths, config_externalStorageDescriptions and
config_mtpReserveSpaceMegabytes resources with an XML resource file
to describe the external storages that are available.

Add android.os.storage.StorageVolume class

StorageManager.getVolumeList() now returns an array of StorageVolume

Change-Id: I06ce1451ebf08b82f0ee825d56d59ebf72eacd3d
Signed-off-by: Mike Lockwood <lockwood@android.com>