fs_mgr: introduce fs_mgr_format to format wiped partitions

Move fastboot's format logic into fs_mgr, to consolidate the knowledge
about how to do this (and when to wipe metadata).

Try to format these formattable paritions if they are wiped.

If formatting fails, we will fall out to let recovery mode handle it.

Bug: 20082763
Change-Id: I397cc197550e78d932e8a154fd234695c46dbe7b