Fix missing abi filter configuration #224

[wuan]

In addition we now have a generic signing configuration. The file key.properties needs to go into android/.

Code Review Checklist

• Description accurately reflects what changes are being made.
• Description explains why the changes are being made (or references an issue containing one).
• The PR appropriately sized.
• New code has enough tests.
• New code has enough documentation to answer "how do I use it?" and "what does it do?".
• Existing documentation is up-to-date, if impacted.

[donpui]

just to be extra careful, I wanted to push this file out of project directory as it contains sensitive information

[wuan]

The local .gitignore already contains a line which should avoid checking in the file:

...
# Remember to never publicly share your keystore.
# See https://flutter.dev/docs/deployment/android#reference-the-keystore-from-the-app
key.properties
...