Added clean task to smoke tests. (#527)
Added clean task to smoke tests. (#527)

This change allows the smoke tests to clean all build variants created
by the infrastructure.
firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk,firebase/firebase-android-sdk