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
Fix building without SELINUX

since the scon is now a mandatory arg, it needs to exist even if
NULL

Change-Id: I8ba31a7a8cf357e9b0c43cee81ab49d3eb192819