aconfig: add new codegen mode force_read_only

The "force_read_only" mode is designed for libraries shared among
multiple containers for the following two scenarios where:
1. The library must be built with read_only flags or
2. Different containers requires different values for the same flags.

In this mode, the library includes only non-exported flags, adhering to
the concept that exported flags should maintain consistent values across
containers. Additionally, regardless of their original permissions, all
flags within this mode will be set to read_only.

Bug: 316357759
Test: atest aconfig.test
Change-Id: I65df39416c96404d84044a46bbcfe1bc8ce9ba8f