Enable 3 button nav for taskbar

Bug: 180046394
Test: Switched between gesture and button nav on
phone and tablet.

Change-Id: Iac2357a3256f8b6b36ca49d3680594a91e824a5f

quickstep/src/com/android/launcher3/taskbar/NavbarButtonsViewController.java

@@ -105,7 +105,7 @@ public class NavbarButtonsViewController {
             return true;
         };
 
-        if (mContext.canShowNavButtons()) {
+        if (mContext.isThreeButtonNav()) {
             initButtons(mStartContainer, mEndContainer, mControllers.navButtonController);
 
             // Animate taskbar background when IME shows