Treat default value that wasn't found as empty value.

Fixes http://code.google.com/p/android/issues/detail?id=33220

Change-Id: If22e8b245f688d922c168aa54c8acf743c0644d7
StatusBar: Add support to show data and wifi activities.

-- Show data activity icons.
-- Show wifi activity icons.

Conflicts:
	packages/SystemUI/res/layout/signal_cluster_view.xml
	packages/SystemUI/src/com/android/systemui/statusbar/SignalClusterView.java
	packages/SystemUI/src/com/android/systemui/statusbar/policy/NetworkControllerImpl.java