Set ANDROID_SDK environment variable in instructions

Added export for ANDROID_SDK environment variable.

Author: pareenaverma

content/learning-paths/mobile-graphics-and-gaming/build-llama3-chat-android-app-using-executorch-and-xnnpack/6-build-android-chat-app.md

@@ -17,6 +17,7 @@ In this section, you will use a Android demo application to demonstrate local in
     ``` bash
     export ANDROID_NDK=$ANDROID_HOME/ndk/29.0.14206865/
     export ANDROID_ABI=arm64-v8a
+    export ANDROID_SDK=$ANDROID_HOME
     ```
 
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