Add ability to run Android tests against multiple API levels

[marcprux]

From the suggestion at apple/swift-collections#558 (comment):

One elaboration to the official Android workflow that would be useful is to be able to specify multiple target triples to run the Android emulator with. That way, we could test the same Swift package against multiple Android API levels, which will be particularly useful once we roll out the new #available(Android) level feature for the SDK.

It might require downloading multiple emulator images, so may be more work to set up, but some Swift packages, like Testing, would benefit from it.

[marcprux]

Can someone please assign this to me and add it to the Swift on Android project with "Todo" status?

[marcprux]

This might warrant overhauling how we handle the android_sdk_triples input parameter. The current behavior is to just build each element of the array serially in a single matrix run, but we might want to change this to create separate jobs for each element, like the android_sdk_versions and android_ndk_versions inputs.

Additionally, I wonder if we should have an option for testing against a different emulator API level than the compile API. E.g., build a package against API 28 but run the tests it against an emulator with API 31.

[finagolfin]

The way I imagined it working is to add another config option android_sdk_test_triples, which would be checked to be a subset of android_sdk_triples, and then we could spin up a separate job for each of the test triples. If we ever get the emulator working on the linux ARM runners once they support nested virtualization, we could spin those up too.