WhisperPush : Fix crash when going in background

When the activity is not in the foreground mid registration, the
service crashes, since the builder was never initialized.

Another issue is, when the activity is in the background, the notification
is never updated since all ui updates are sent to the activity instead.

This patch resolves both issues

Change-Id: I89f78b8bf357bc19c1aa1ebe8b81fc7e8eea48a7
(cherry picked from commit 0f413b918d570fd060bcb10dee24c563166280cf)