Update Github Actions to use Ubuntu for Android

[seljabali]

Issue: https://github.com/touchlab/KaMPKit/issues/236

Summary

Using macOS machines for building/testing Android modules is not a good bet for the following reasons:

• macOS machines, I found, are slower than their Ubuntu counterparts.
• macOS machines cost 10x more in billing. Those who clone this repo privately will pay the price unnecessarily.

Fix

Made Github Actions faster & less costly.

Testing

• ./gradlew :app:build
• ./gradlew :shared:build
• xcodebuild -workspace ios/KaMPKitiOS.xcworkspace -scheme KaMPKitiOS -sdk iphoneos -configuration Debug build -destination name="iPhone 8"
• manual testing

[russhwolf]

This is ok for Android, but the shared build is more complicated. Running the shared build on ubuntu means it won't run the shared tests on the ios simulator, and we don't want to lose that.

[seljabali]

Reverted shared module to continue using macOS.

[kpgalligan]

Thanks!