Remove implementation details from stub flags in sdk snapshot

Previously, the build applied the same filtering to remove
implementation details from the sdk snapshot's stub-flags.csv file as
it did for its all-flags.csv, i.e. removing the signatures that only
had a "blocked" flag. Unfortunately, that had no effect on the stub
flags as the implementation signatures had no flags, not a single
blocked flag. That meant that the sdk snapshot's
filtered-stub-flags.csv file contained a lot of implementation details.

This change removes signatures from stub-flags.csv that have no flags
which removes all implementation details from the sdk snapshot.

Bug: 194063708
Test: atest --host verify_overlaps_test
      m out/soong/hiddenapi/hiddenapi-flags.csv
      m art-module-sdk
      # Check contents of its filtered-stub-flags.csv file
Change-Id: I30edc77348fad118ea732e787ae8e206c8841f84