Normalize the path that ANDROID_HOME uses (#28222)

The gradle/android tooling will validate that ANDROID_HOME and
ANDROID_SDK_ROOT are precisely the same. So if they point to the same
directory but are not normalized this check will fail.

Fixes Dart-Flutter Head-Head-Head bot which failed with
```
Several environment variables and/or system properties contain different paths to the SDK.
  Please correct and use only one way to inject the SDK location.
  
  ANDROID_HOME: /b/s/w/ir/cache/builder/engine/src/flutter/testing/rules/../../../third_party/android_tools/sdk
  ANDROID_SDK_ROOT: /b/s/w/ir/cache/builder/engine/src/third_party/android_tools/sdk
```
1 file changed
tree: aa18bc85685de18675da524026aa51cff32e5f64
  1. .github/
  2. assets/
  3. benchmarking/
  4. build/
  5. ci/
  6. common/
  7. docs/
  8. examples/
  9. flow/
  10. flutter_frontend_server/
  11. fml/
  12. lib/
  13. runtime/
  14. shell/
  15. sky/
  16. testing/
  17. third_party/
  18. tools/
  19. vulkan/
  20. web_sdk/
  21. .ci.yaml
  22. .cirrus.yml
  23. .clang-format
  24. .clang-tidy
  25. .gitattributes
  26. .gitignore
  27. analysis_options.yaml
  28. AUTHORS
  29. BUILD.gn
  30. CONTRIBUTING.md
  31. DEPS
  32. Doxyfile
  33. LICENSE
  34. README.md
README.md

Flutter Engine

Build Status - Cirrus

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