Update play services to 15.0.0

Change-Id: Ia416b5237d98bc09952828930a1638436c3955a6

Author: samtstern

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'