aconfig: add fixed read only flag

Add a new field in the declaration to indicate whether
the permission can be overridden.

When the field “is_fixed_read_only” is set to true,
the flag permission will be set as fixed “READ_ONLY”,
and the permission should not be changed by Gantry.

Bug: 292521627
Test: atest aconfig.test
Change-Id: Ic9bcd7823bccb8b947cf05568c7ced3763490a23