Fix systemui crash on boot

The process com.android.systemui would crash after phone booting with a
java.lang.NullPointerException when the signal status style is set to text

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/SignalClusterTextView.java

Change-Id: I43b33f035497c2d43c688b408f10fb8b348b3b96