Require prebuilt_bootclasspath_fragment to provide hidden API files

Previously, the prebuilt_bootclasspath_fragment was not required to
provide hidden API files as not all usages of that module provided
them. That should no longer be the case so this change makes the
prebuilt_bootclasspath_fragment stricter and so now always require
them.

Bug: 194063708
Test: m out/soong/hiddenapi/hiddenapi-flags.csv
      - check that this does not change the contents.
Change-Id: I3f19a1b4246f09a927c0f6b0f41b8678150d6cc2