Don't assign ANDROID_HOME if set already (pytorch#15198)

based on
https://github.com/pytorch/executorch/blob/4bf7e12cce28c2a9695bb34a0028f3cf25c9b829/docs/source/using-executorch-android.md
users might set up ANDROID_HOME by themselves. However the current
script doesn't use that. Fix the script and have it users the one user
specified

Author: cccclai

scripts/build_android_library.sh

@@ -82,9 +82,9 @@ build_aar() {
     find cmake-out-android-so -type f -name "*.so" -exec "$ANDROID_NDK"/toolchains/llvm/prebuilt/*/bin/llvm-strip {} \;
   fi
   pushd extension/android/
-  ANDROID_HOME="${ANDROID_SDK:-/opt/android/sdk}" ./gradlew build
+  ANDROID_HOME="${ANDROID_HOME:-${ANDROID_SDK:-/opt/android/sdk}}"
   # Use java unit test as sanity check
-  ANDROID_HOME="${ANDROID_SDK:-/opt/android/sdk}" ./gradlew :executorch_android:testDebugUnitTest
+  ANDROID_HOME="${ANDROID_HOME:-${ANDROID_SDK:-/opt/android/sdk}}" ./gradlew :executorch_android:testDebugUnitTest
   popd
   if [ ! -z $BUILD_AAR_DIR ]; then
     cp extension/android/executorch_android/build/outputs/aar/executorch_android-debug.aar "${BUILD_AAR_DIR}/executorch.aar"