Fixed UserSwitchingDialog theme.

It didn't set windowAnimationStyle, and hence
ViewTreeObserver.OnWindowShownListener.onWindowShown() was never called.

Fixes: 145021634
Bug: 145211062
Test: manual verification switching users

Change-Id: I7f995dbe21922422f678f460d69fa37a3b4357d1
(cherry picked from commit 2a30fe59626060277398f93aaf754a12b873d7d0)