Make the dependency list customizable.

[freakboy3742]

Makes the list of libraries a templated variable, rather than a hard-coded list.

Removes the testImplementation and androidTestImplementation entries on the basis that Toga apps won't be using them.

Also makes the base theme configurable; this is needed because if you're using Material Components, you need to use a Material base theme, or most of your Material widget colors will default to Black.

Bumps the target SDK version to 34, so that we can use a recent version of Material Components.

Briefcase-Repo: https://github.com/freakboy3742/briefcase.git
Briefcase-Ref: android-libs

PR Checklist:

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

[mhsmith]

Not merging yet, because it'll break the main branch until beeware/briefcase#1610 is merged too.