Add support for passthrough args

[mhsmith]

Supports https://github.com/beeware/briefcase/pull/1078. The template PR should be merged before the Briefcase one.

PR Checklist:

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

[mhsmith]

TBD: verify that it's not possible to append to sys.argv more than once in a single process.

[mhsmith]

I was able to get it to add the args multiple times by doing the following on API level 24:

• Start the app with Briefcase
• Press Back
• Return to the app using the app switcher (square button)

Fixed by moving the args code into the if branch which starts Python.