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

从链接的 Buildkite Android 测试失败开始，检查 Bazel@HEAD 日志中缺失的输入 external/androidsdk/build-tools/28.0.2/lib/dx.jar。读取所引用缓存路径中的 external/androidsdk/BUILD.bazel，并比较 Android SDK 依赖配置。当 Android Testing CI 在所有受影响的平台上通过且不再出现缺失 dx.jar 错误时，即表示完成。

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