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
```
diff --git a/testing/rules/run_gradle.py b/testing/rules/run_gradle.py
index 2ca3f0f..bb625fe 100644
--- a/testing/rules/run_gradle.py
+++ b/testing/rules/run_gradle.py
@@ -14,12 +14,15 @@
 import platform
 
 SCRIPT_PATH = os.path.dirname(os.path.realpath(__file__))
-ANDROID_HOME = os.path.join(SCRIPT_PATH, '..', '..', '..', 'third_party', 'android_tools', 'sdk')
+ANDROID_HOME = os.path.normpath(os.path.join(SCRIPT_PATH, '..', '..', '..',
+    'third_party', 'android_tools', 'sdk'))
 
 if platform.system() == 'Darwin':
-  JAVA_HOME = os.path.join(SCRIPT_PATH, '..', '..', '..', 'third_party', 'java', 'openjdk', 'Contents', 'Home')
+  JAVA_HOME = os.path.normpath(os.path.join(SCRIPT_PATH, '..', '..', '..',
+      'third_party', 'java', 'openjdk', 'Contents', 'Home'))
 else:
-  JAVA_HOME = os.path.join(SCRIPT_PATH, '..', '..', '..', 'third_party', 'java', 'openjdk')
+  JAVA_HOME = os.path.normpath(os.path.join(SCRIPT_PATH, '..', '..', '..',
+      'third_party', 'java', 'openjdk'))
 
 def main():
   if not os.path.isdir(ANDROID_HOME):