Android: push trunk back to API 24, stop rebuilding corelibs for the linux host, and update trunk CI to NDK 28c in preparation for the upcoming LTS NDK 30 release

[finagolfin]

The Android workgroup agreed on pushing the CI and SDK back to be built against API 24, so this pull does so for trunk alone, as we're still waiting on a pull for 6.3, swiftlang/swift-corelibs-foundation#5398.

I also added some flags to avoid rebuilding some corelibs for the linux host, as we currently invoke build_script once for each Android arch, and that script deletes and rebuilds the previously built corelibs by default each time, so this makes sure we don't do that.

The real solution is to add a build flag to avoid building the corelibs for the host altogether when building cross-compilation toolchains like this, so I'm working on a handful of pulls for that and to slim down the number of repos checked out for these SDK builds, but this pull will do in the meantime.

What does the CI team think about adding some github CI for these Android pulls, so we can check them before merging? I'm working on an external github CI, swift-android-sdk#19, that will run much more often than this official CI and alert me to problems much more quickly, but that wouldn't be as useful for new contributors that would have to find that repo first.

Also, it would be good if we could add some compiler caching to this official Android CI and set it up so it runs on every compiler pull before merging, though this new Android CI job would not be required to pass yet for those pulls to be merged.

[finagolfin]

Rebased and updated this to NDK 28c for trunk, looking into a fix for the duplicated swiftmodules next, #510, but found the reason.