Mms: Add native heads up support

Add heads up support (optional but enabled by default).

Shows important messages of this app as heads up.

Means:
- All MMS
- SMS if quickpopup is disabled

Other messages like storage or blacklist notifications are not important and will not show as heads up

PS:
- show always as expanded.....so user can directly interact with it

Change-Id: Id83633610c18ab4e380bfb910ea50e8bf7b61be6
Signed-off-by: scott <scott@ubuntu>

Conflicts:
	res/values/slim_strings.xml
	res/xml/preferences.xml
	src/com/android/mms/ui/MessagingPreferenceActivity.java