Android testing CI faliing (all platforms)

Android Testing CI has been failing for many days, and thus it's been difficult to diagnose issues with it failing with Bazel@HEAD

[Here](https://buildkite.com/bazel/bazel-at-head-plus-downstream/builds/1368#3a7fa3e7-5af6-4902-aba9-032104d467a8) is a failure for Unbuntu 16 this morning using Bazel@HEAD:
```
(02:09:03) ERROR: missing input file 'external/androidsdk/build-tools/28.0.2/lib/dx.jar', owner: '@androidsdk//:build-tools/28.0.2/lib/dx.jar'
--
  | (02:09:03) ERROR: /var/lib/buildkite-agent/.cache/bazel/_bazel_buildkite-agent/d3e6522be4da9d904ab54b448eb64f5e/external/androidsdk/BUILD.bazel:8:1: @androidsdk//:dx_jar_import: missing input file '@androidsdk//:build-tools/28.0.2/lib/dx.jar'
```

I think this dependency was updated in a recent commit?

https://buildkite.com/bazel/android-testing

Start with the linked Buildkite Android testing failure and inspect the Bazel@HEAD logs for the missing external/androidsdk/build-tools/28.0.2/lib/dx.jar input. Read external/androidsdk/BUILD.bazel at the referenced cache path and compare the Android SDK dependency configuration. Done means the Android Testing CI passes across the affected platforms without the missing dx.jar error.

Written by the indexing model from the issue text.