SystemUI: Don't try to update the navbar if it doesn't exist

Fix crashes on theme change on devices with buttons

Change-Id: I117c0ae605f7caca7e0f0425b80645a985644630

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

 Author:    Ricardo Cerqueira <cyanogenmod@cerqueira.org>
 On branch jb422
 Changes to be committed:
	modified:   packages/SystemUI/src/com/android/systemui/statusbar/phone/PhoneStatusBar.java