fs_mgr: store file_contents_mode and file_names_mode directly

It doesn't really make sense to have extra logic to convert these
strings to enums then back again to strings for usage, especially
since with the C++ fstab, these strings are small enough to fall into
the small string optimization of std::string.

This will help make future changes cleaner as well.
Test: boot, fs_mgr_test

Change-Id: I5669ed10f2fc3eafdb137747446a2e93c24d55c4