Properly null check all button usages.

Users might have hidden them.

Change-Id: I7f52d570cf4ef0833c6bde9159475345d581a00f

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

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/phone/NavigationBarView.java
navbar fix

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/phone/NavigationBarTransitions.java
Revert "[1/3] Flip to Mute/Reject Call"

This reverts commit d6018c0f39bba1d58dc121671718413d9ee3febb.
Revert "[1/2] Base: Network traffic indicator"

This reverts commit e682adfe8c2ea1179fbbcd6598b9ff3de56b75d9.