aconfig: add new testing flag enabled_fixed_ro_exported

This commit adds a new testing flag enabled_fixed_ro_exported to test
the case of a exported and fixed_read_only flag.

Test: atest aconfig.test aconfig.test.java AconfigJavaHostTest
aconfig.test.cpp aconfig.test.cpp.test_mode aconfig.prod_mode.test.rust
Bug: 316357680

Change-Id: Iaedb8a6875166c6a6d24c7c3deee701a496b4964