support converting between f2fs <-> ext4 format type

Under "mounts and storage" menu, if "toggle f2fs <-> ext4 migration" is enabled, whenever we try to format an ext4 or f2fs partition, we get a prompt to convert between f2fs/ext4
To convert /data on /data/media devices, we must as always select the "/data and /data/media" menu

Nandroid implementation to migrate your data/ROM from ext4 to f2fs:
- backup your existing data
- in "mounts and storage", format the partitions you want to convert to f2fs
- restore your data

On restore, if both backup image and target partition has different fstype and fstype is either ext4 or f2fs, we favour target devise fsytpe (data migration)

Many parts of the code were adapted from @kumajaya work on f2fs

Change-Id: I573342fcdb844a3cb2415e19d498228334c0aca3