Fix NPE in StatusBar#onBackPressed()

StatusBar.mKeyguardUserSwitcher is instantiated
only if multi user function is enabled.
But mKeyguardUserSwitcher.hideIfNotSimple() is invoked
without null check in StatusBar#onBackPressed().
This causes NPE on multi user disabled devices.

Fixes: 65391930
Test: manual - tap Back key on lockscreen

Change-Id: I528823fa832ee5096d73dea660997fc276d86c5c

packages/SystemUI/src/com/android/systemui/statusbar/phone/StatusBar.java

@@ -4486,7 +4486,7 @@ public class StatusBar extends SystemUI implements DemoMode,
             animateCollapsePanels();
             return true;
         }
-        if (mKeyguardUserSwitcher.hideIfNotSimple(true)) {
+        if (mKeyguardUserSwitcher != null && mKeyguardUserSwitcher.hideIfNotSimple(true)) {
             return true;
         }
         return false;