Filter blocked entries from modular flag files

Previously, the sdk snapshot would include all the entries from the
stub-flags.csv and all-flags.csv modular files generated by a single
bootclasspath_fragment. That included implementation details, i.e.
class members that are not part of a stable API or the hidden API which
meant that the sdk snapshots were implementation dependent.

This change removes the implementation details from the modular flag
files, i.e. those entries that are only marked as "blocked". When
comparing the files against the corresponding subset of the monolithic
files it assumes that any entries missing from the modular flag files
are blocked.

Bug: 194063708
Test: atest --host verify_overlaps_test signature_patterns_test
      m out/soong/hiddenapi/hiddenapi-flags.csv
      - manually change files to cause difference in flags to check
        that it detects the differences.
Change-Id: I6b67b2253cf029d6830b58a06ebb0c8fcaa0dd71