Begin development on v4.0.0

Change-Id: I794f2adf56b5ac8e08217c4d4a6ac7aba96c5f4b

Author: samtstern

constants.gradle

@@ -1,7 +1,7 @@
 project.ext {
     submodules = ['database', 'auth', 'storage', 'firestore', 'common']
     group = 'com.firebaseui'
-    version = '3.4.0-SNAPSHOT'
+    version = '4.0.0-SNAPSHOT'
     pomdesc = 'Firebase UI Android'
 
     compileSdk = 27