Improve flexibility of Android emulator system image

The Android toolchain currently hard-codes the system image that is used on emulator images built by Briefcase (at time of writing, the default Android-31 image is used). This system image is downloaded when the emulator binary is downloaded, and is never directly updated.

This has some unfortunate side effects:
1. If a user has an emulator created by a different set of tools (e.g., Android Studio), the emulator will appear in the list of available emulators, but won't be able to start
2. A user can't intentionally choose a different base image - e.g., to test older/newer devices.
3. A user can't intentionally choose a base image other than the Default image (e.g., a Google Play enabled image)

**Describe the solution you'd like**

It would be desirable for Briefcase to:
1. provide the option of selecting a base image from the list provided by the SDK manager
2. check that the system image required by an emulator is available before trying to start the image

**Describe alternatives you've considered**

There aren't really any alternatives, other than manually invoking sdkmanager to create images and download system images that meet your requirements.

**Additional context**

`sdkmanager` is a tool provided by the Android toolchain to do package management. Run `briefcase run -v` to see examples of how it can be run, including the environment variables that must be set in order to invoke it.

`sdkmangager --list` will give you a list of available images; any package starting with `system_images;` is, as the name suggests, a system image. The default system image used by briefcase on M1 hardware is `system-images;android-31;default;arm64-v8a` (system images are architecture dependent; only architecture-appropriate options should be provided).

`sdkmangager --list_installed` will give you a list of images that are currently installed.

`sdkmanager ` will download and install a package.

Android emulators (called AVDs) are stored in `~/.briefcase/avd`. There is a `.ini` config file and a `.avd` directory for each emulator, with the name of these files matching the emulator name. The `.avd` folder contains a `config.ini` that details the system image required by the AVD under the `image.sysdir.1` key (e.g., `image.sysdir.1=system-images/android-31/default/arm64-v8a/`)

Start with `briefcase run -v` to review how the Android toolchain invokes `sdkmanager`, then inspect the AVDs under `~/.briefcase/avd`, including each `.avd/config.ini` and its `image.sysdir.1` value. Use `sdkmanager --list` and `--list_installed` to understand available and installed system images. Done means users can choose an architecture-appropriate image and Briefcase checks that an AVD's required image is available before starting it.

Written by the indexing model from the issue text.