Update play services to 15.0.0 (#1254)

Author: samtstern

auth/src/test/java/com/firebase/ui/auth/testhelpers/AutoCompleteTask.java

@@ -59,6 +59,11 @@ public boolean isSuccessful() {
         return mSuccess;
     }
 
+    @Override
+    public boolean isCanceled() {
+        return false;
+    }
+
     @Override
     public TResult getResult() {
         if (mSuccess) {

constants.gradle

@@ -8,7 +8,7 @@ project.ext {
     targetSdk = 27
     minSdk = 14
 
-    firebaseVersion = '12.0.1'
+    firebaseVersion = '15.0.0'
     supportLibraryVersion = '27.1.0'
     architectureVersion = '1.1.0'
     kotlinVersion = '1.2.30'