Add Android support

[stackotter]

This is a WIP. I'll fill out the description once things are bit more complete.

Todo

• Resolve Android Swift SDK path ourself, cause SwiftPM isn't good at choosing when there are multiple Android SDKs installed, and it doesn't ensure that the SDK was built with the toolchain currently being used. If we also implement Swift toolchain discovery, then we could probably automatically locate valid toolchain/Android Swift SDK combinations in most valid setups.
• Support metadata insertion on Android (requires being able to locate SDKs ourselves).
• APK bundling
• Make target Android API version configurable
• Change target architecture to match target device
• Support multi-arch Android APK builds
• Automatically locate compatible Swift toolchains (because Swift Android SDKs are quite particular)

Future improvements

• Update toolchain compatibility resolution to be two-way (i.e. if you provide a toolchain but no SDK, then we should take the toolchain into account when selecting an SDK)
• Choose between multiple matching SDKs by eliminating any that don't have a corresponding compatible toolchain installed (rather than just picking the first one)