Cleanup signature_patterns*.py

Fix issues reported by pylint, Intellij Python checks and also try and
adhere to the Google Python Style Guide.

Bug: 194063708
Test: atest signature_patterns_test
      m out/soong/hiddenapi/hiddenapi-flags.csv
      /usr/bin/pylint --rcfile $ANDROID_BUILD_TOP/tools/repohooks/tools/pylintrc scripts/hiddenapi/signature_patterns*.py
      pyformat -s 4 --force_quote_type single -i scripts/hiddenapi/signature_patterns*.py
Change-Id: I64d64e9cb269f58d65f4e10ec2f0e874154919e6