Bring back translucent Navbar and fix it

This reverts commit 2246fbc72b5c04f8e1a8647e5e907a043974feb7.

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/phone/BarTransitions.java
	packages/SystemUI/src/com/android/systemui/statusbar/phone/NavigationBarView.java
Avoid SysUI force closing when enable/disable DSB System bar gradient option

Change-Id: I5d6dcea9f4d7187a9c14d8ef0dd5c1c874cb8d9f