fs_mgr_fstab: allow fileencryption without equals sign

A point of confusion that has been encountered recently is that the
fileencryption argument is optional (since Android 11), yet the fstab
parser requires the equals sign in "fileencryption=".  Thus, someone
wanting to use the default options must use "...,fileencryption=,...".

Make "fileencryption" by itself mean the same thing so that it works as
expected.

Test: atest CtsFsMgrTestCases
Change-Id: I65ce6b9513942bec2107838396835e7aafb3bf37