Create prebuilt system modules for all API scopes

Previously, system modules were only created for the public API scope.
This change creates them for any API scope as long as its directory has
a core-for-system-modules.jar.

It does that by hooking into the existing logic for creating a
java_import for all jars in the API directories and creating a
java_system_modules for every core-for-system-modules.jar file. That
avoids the need for extra path globs.

Test: m droid
      m sdk_public_current_system_modules
      - works as expected.
      m sdk_module-lib_current_system_modules
      - fails with missing target as expected.
      touch prebuilts/sdk/current/module-lib/core-for-system-modules.jar
      m sdk_module-lib_current_system_modules
      - fails with invalid jar file as expected.
Bug: 204189791
Change-Id: I27a264941009e03439d5d847dab14a7b4f6f119f