Extract a subset of the monolithic flags for comparison

Previously, the compare_signature_flags compared the full set of
monolithic flags against the modular flags and just ignored any
monolithic flags for a signature that was not present in the modular
flags. It essentially performed two tasks, identifying the subset of
the monolithic flags against which the modular flags should be compared
and then doing the actual comparison.

This change separates those two tasks, by first creating a dict that is
the subset of the monolithic flags against which the modular flags will
be compared and then do the comparison.

At the moment the subset is simply the set of monolithic flags
associated with the signatures that are present in the modular flags.
Follow up changes will provide other ways to select the subset.

Bug: 194063708
Test: atest --host verify_overlaps_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: Icb547f071b814dd955f31255b0d35597f7ba4aa1