aconfig: add ForceReadOnly mode to aconfig c/c++ codegen

This commit adds ForceReadOnly mode to c/c++ codegen.

Bug: 316357759
Test: atest aconfig.test aconfig.test.cpp aconfig.test.cpp.test_mode
aconfig.test.cpp.exported_mode aconfig.test.cpp.force_read_only_mode

Change-Id: I4842dd69993fe4fc1f504358f59513cf064919d9