Begin development on v4.0.1

Change-Id: I919736b08fa499c63260a9b226f3fd2a0a8383ab

Author: samtstern

constants.gradle

@@ -1,7 +1,7 @@
 project.ext {
     submodules = ['database', 'auth', 'storage', 'firestore', 'common']
     group = 'com.firebaseui'
-    version = '4.0.0'
+    version = '4.0.1-SNAPSHOT'
     pomdesc = 'Firebase UI Android'
 
     compileSdk = 27