UnifiedEmail: Add native heads up support per account (2/2)

Add native heads up notification support. User can set on every account
if he or she want this account as heads up notification or not.

This patch adds the settings for it as well the complete handling. Due
of a half broken NotificationCompat.Builder class we switch here to the
Notification.Builder class to get access to the methods we need. This is no
problem here due that we do not need compatibility on lower API's

Change-Id: I32cd1b914777dd3a491ab20fe681d347a9d81e3c