Active display: Use a default notificaiton icon.

In case there is an exception when trying to load the
notification icon we will use a default one.

Change-Id: I4087ef3e407cf976f7bfb2bb89f850641101bc73

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/policy/activedisplay/ActiveDisplayView.java

	modified:   core/java/android/provider/Settings.java
	new file:   packages/SystemUI/res/drawable-hdpi/ic_ad_unknown_icon.png
	new file:   packages/SystemUI/res/drawable-mdpi/ic_ad_unknown_icon.png
	new file:   packages/SystemUI/res/drawable-xhdpi/ic_ad_unknown_icon.png
	modified:   packages/SystemUI/src/com/android/systemui/statusbar/policy/activedisplay/ActiveDisplayView.java
add hide low prioity notifications
	modified:   core/java/android/provider/Settings.java