Normalize the path that ANDROID_HOME uses (#28222)
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
```
flutter/engine,devoncarew/engine,Hixie/sky_engine,jason-simmons/flutter_engine,rmacnak-google/engine,rmacnak-google/engine,rmacnak-google/engine,aam/engine,devoncarew/engine,devoncarew/engine,aam/engine,devoncarew/engine,chinmaygarde/flutter_engine,chinmaygarde/flutter_engine,Hixie/sky_engine,rmacnak-google/engine,devoncarew/engine,jason-simmons/flutter_engine,Hixie/sky_engine,devoncarew/engine,aam/engine,Hixie/sky_engine,Hixie/sky_engine,jason-simmons/flutter_engine,aam/engine,chinmaygarde/flutter_engine,Hixie/sky_engine,Hixie/sky_engine,flutter/engine,jason-simmons/flutter_engine,rmacnak-google/engine,aam/engine,aam/engine,jason-simmons/flutter_engine,flutter/engine,flutter/engine,jason-simmons/flutter_engine,flutter/engine,flutter/engine,flutter/engine,Hixie/sky_engine,devoncarew/engine,jason-simmons/flutter_engine,chinmaygarde/flutter_engine,rmacnak-google/engine,aam/engine,aam/engine,chinmaygarde/flutter_engine,chinmaygarde/flutter_engine,rmacnak-google/engine,chinmaygarde/flutter_engine,flutter/engine,jason-simmons/flutter_engine