fix package of android example

[niucho]

fix package of MainActivity.kt in sample

• error logs

e: /Users/user/Projects/client-sdk-flutter/example/android/app/src/main/kotlin/com/example/livekit_example/MainActivity.kt: (5, 7): Redeclaration: MainActivity
e: /Users/user/Projects/client-sdk-flutter/example/android/app/src/main/kotlin/io/livekit/example/MainActivity.kt: (5, 7): Redeclaration: MainActivity

FAILURE: Build failed with an exception.

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[cloudwebrtc]

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