Separate creation of signature patterns from overlap checking

Previously, the signatures used to select the subset of the monolithic
flags were simply the signatures read from the modular flags file. This
change moves the creation of the signature list into a separate script
that outputs the signatures to a file and then passes the path through
Soong from the bootclasspath_fragment modules that create it to the
platform_bootclasspath module that uses it to compare the modular
flags against the monolithic flags.

Currently, the signatures are the full signatures but follow up changes
will replace them with patterns (hence the name) that avoids having to
include implementation details in the hidden API flags that are output
as part of a bootclasspath_fragment's snapshot.

This change moves the stub flags related code next to the all flags
related code as they are treated in a similar way.

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: I2855bf6d05c91b8a09591664185750361c7e644f