Allow module SDK snapshot build to hide flagged APIs

Temporary workaround to allow module SDK snapshot builds to hide
flagged APIs until such time as development moves to `main` and
the build in `main` can handle that properly.

Bug: 299570421
Test: UNBUNDLED_BUILD_SDKS_FROM_SOURCE=true vendor/google/build/mainline_modules_sdks.sh
      # Check that flagged APIs are excluded from everything but
      # for-latest-build.
Change-Id: I5b133b98aab91b2ad5ba717dd663c60a5ed90c9d
Merged-In: I4967376c0236bad729398af80fa59b48dbab5f21