fs_mgr: fix crash on "noemulatedsd" in fstab

MF_FORCECRYPT claimed same flag value as existing MF_NOEMULATEDSD.
MF_FORCECRYPT also crashes if no "=" in the option.

Modify MF_NOEMULATEDSD flag value to unique, and reformat comment to keep
the definition near the other defines.

Change-Id: Iec86f40ff43eea14b791d3d19c0ae11fd4b5f784