android/ci: run CI jobs on Linux

Make all Android CI jobs use Linux as opposed to the combination of both Linux and macOS.

Currently some of the Android CI jobs are ran on Linux (`android_tests / kotlin_tests_linux`) while the other are ran on macOS (`android_tests / kotlin_tests_mac`, `android_tests / java_tests_mac`, `android_build / kotlin_helloworld`).

Building Envoy Mobile aar and running example apps in Android emulator requires a full Android toolchain - that's something that may need to be set up on our Linux machines before we are able to migrate over to them.