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

(cherry picked from commit 11bbdebfd01f576c9dbe2c3754f753a856ec8aa3)
UnifiedEmail: change to switches

Change-Id: I2e0fa8bcc774b8a02cc2db93c4418756926764d9

(cherry picked from commit d3ad9e17d93958bef8071c65f2a9f235c32b105b)
UnifiedEmail: be sure if BdiFormatter is null to get the instance instead.

Otherwise it ends up in an NPE.

Change-Id: Icae9cacb7a7e2c9cdcafda27ed6eec347324b499

(cherry picked from commit abf3723d8f2124256225f0364f5e7e18d2a08720)