Export signature patterns to sdk snapshot

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: Ic819def9b0c41e3b22fb98582cd39f6d27080a32