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