Added org.beeware.android.SpinnerArrayAdapter

[t-arn]

This PR adds the SpinnerArrayAdapter class which is needed to implement Toga's Selection.set_font() for Android

PR Checklist:

• [x] All new features have been tested
• [ ] All new features have been documented
• [x] I have read the CONTRIBUTING.md file
• [x] I will abide by the code of conduct

[t-arn]

@freakboy3742 Please review this PR

[freakboy3742]

As discussed in my review on beeware/toga#1758, adding stubs to the app template isn't a viable path forward.