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
UnifiedEmail: change to switches

Change-Id: I2e0fa8bcc774b8a02cc2db93c4418756926764d9
UnifiedEmail: be sure if BdiFormatter is null to get the instance instead.

Otherwise it ends up in an NPE.

Change-Id: Icae9cacb7a7e2c9cdcafda27ed6eec347324b499