Changed CarUserSwitching UI to show which user is loading.

This option is only available on debuggable builds, and it makes it easier
to debug slow user-switching issues.

Bug: 145027829
Bug: 145915635
Test: manual verification

Change-Id: I10bf6d5fe3cf17480568d29e6407506522bf7679
(cherry picked from commit 81b79b124d3d9a3a0fd92066f9708e05185fe628)