java.lang.NoClassDefFoundError: Failed resolution Failed resolution of: Lcom/google/android/gms/common/internal/zzab;

now here is the problem, you are using different versions in dependencies of gradle So change this

compile 'com.google.firebase:firebase-database:10.0.0'
testCompile 'junit:junit:4.12'
compile 'com.firebase:firebase-client-android:2.3.1'
compile 'com.google.firebase:firebase-auth:9.2.1'
compile 'com.google.firebase:firebase-storage:9.2.1'
compile 'com.android.support:multidex:1.0.0'

to this

compile 'com.google.firebase:firebase-database:10.0.1'
testCompile 'junit:junit:4.12'
compile 'com.firebase:firebase-client-android:2.3.1'
compile 'com.google.firebase:firebase-auth:10.0.1'
compile 'com.google.firebase:firebase-storage:10.0.1'
compile 'com.android.support:multidex:1.0.0'

one more thing is that now com.firebae:firebase-client-android is deprecated so you need to replace it with com.google.firebase:firebase-database:10.0.1 and here is the documentation for that https://firebase.google.com/support/guides/firebase-android

I have faced similar problem while implementing native ad and killed almost 4 hours...

At last i have figured out that there are different com.google.android.gms:play-services-ads version in different module('17.1.1' and '12.0.0').

After changing all version to 17.1.1 it solved my problem.