SystemUI: Do not use missing status bar icons

This commit simply makes sure the status bar elements that are not
found are not interacted with. The battery indicator icons were
already partially taken care of so this commit simply provides
a more complete safeguard.

Change-Id: Ic9cd154caf29d27f93a3c5c0598a826eb31165bf

Conflicts:

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