Revert "remove navigation bar customization"

This reverts commit 4b58b80587a3e562bcc516940c02c8435a417541.

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/phone/NavigationBarTransitions.java
	packages/SystemUI/src/com/android/systemui/statusbar/phone/NavigationBarView.java
	packages/SystemUI/src/com/android/systemui/statusbar/policy/KeyButtonView.java
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
hopefully get customizable navbar working on 4.4.2