Ensure that no callback is invoked while initializing DatePicker

bug:3360821

1. While my previous change:I3baff68c has partially fixed this bug
   it was still possible for a callback to be invoked on init. If a
   callback was already regitsered and the init is called the
   callback is incorrectly notified.

Change-Id: I05c6cb78f4c7b7d2a00c52aef42c1698d9479be5