Add settings.gradle to prep for Gradle 5 (#391)
Add settings.gradle to prep for Gradle 5 (#391)

Gradle 5 needs this file to distinguish project boundaries.
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