fs_mgr: allow any argument to encryptable

There are some fstab files that specify "encryptable=footer" for
adoptable storage volumes, which contradicts the documentation which
says that it should be "encryptable=userdata".  However, the argument
was previously being ignored anyway.  To avoid unnecessarily breaking
such devices, ignore the argument to "encryptable".

Note that we continue to only allow "encryptable" in combination with
"voldmanaged".  So, fstabs that use "encryptable" for FDE (rather than
for adoptable storage) should continue to be rejected.

Bug: 191796797
Change-Id: Idc4d5f9c01098f997e12be0022bea992439cec9c