aconfig: add invariant checks for `is_fixed_read_only`

Update parsed_flag::verify_fields to verify that the invariant "if a
parsed flag is_fixed_read_only, its permission must be READ_ONLY".

Note: commands.rs will ensure that no value files are used to create an
invalid parsed flag when flag declarations and values are parsed, but
the invariant check should still be added for completeness sake, and to
make sure no invalid hand-crafted test data exists.

Also fix invalid test data found by adding this invariant.

Bug: 298935897
Test: atest aconfig.test
Change-Id: I8cea5c4d28c458525b7034d78ecb34dd09272771