android/ci: run CI jobs on Linux

[Augustyniak]

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.

[Augustyniak]

FYI @danzh2010

[lfpino]

@Augustyniak let us know (Engflow) when you'll start this work to make sure everything works on the remote execution end :)

cc @Yannic

[jpsim]

One way to do this would be to add Android toolchain stuff (and possible Android Studio stuff?) to https://github.com/envoyproxy/envoy-build-tools which is the repo that generates the Docker image we use to run Android CI jobs, both on the GitHub Actions hosts and the Engflow remote executors.

[jpsim]

Part 1: envoyproxy/envoy-build-tools#188