Tablet-specific UI updates to user switcher. DO NOT MERGE

 - Draw a background pill instead of frame on the keyguard
   for the currently active user.
 - Set a different text style for the user switcher

Bug: 146426568
Test: Manual inspection

Change-Id: I5cfefbe375bb0f3b7df8729c473867b97578f81f
(cherry picked from commit d457eef3)