fs_mgr: Drop the context mount option from non-selinux builds

This will require that transitioning devices have context as the
last mount option, since everything after it will be ignored

Change-Id: Ic5cc7f5632cc3d62459dfaaf7719d662737e1641
fs_mgr: Don't assume all fstab entries have options

Change-Id: I5f537b09d96efa2758372f9da6d66996c2164ee8