Add ModuleLibHiddenAPIScope for hidden API processing

Although the hidden API runtime does not support a module-lib API flag
the hidden API processing does need to used them as they are needed by
the "hiddenapi list" tool for any bootclasspath_fragment module whose
contents builds against an sdk_version of "module_current". Without it
the "hiddenapi list" tool could fail to resolve references to classes
provided by other modules which would break the build.

Bug: 179354495
Test: m out/soong/hiddenapi/hiddenapi-flags.csv
      - make sure that this change has no effect on the generated flags.
Change-Id: I3ecb80fdaeba0e66d1ee25cb57152ab546d9bfe0