Stop using --channel=3 (canary) when starting Android emulator in CI

In https://github.com/envoyproxy/envoy-mobile/pull/1695 we started using `--channel=3` option when downloading an Android emulator when running CI jobs. `--channel=3` means canary so we should probably find a way to stop using it at some point. We tried multiple channels (0, 1, 2, 3) but only 3 worked.

This was needed since our Android CI jobs started failing with the following error (without any change on our side):
```
Warning: Package "Android Emulator" with revision at least 30.8.2 not available.
```