[flexiglass] Don't show bouncer when lockscreen is clicked on mobile

We don't want this behaviour on mobile but we still want it on desktop.

Fix: 433950243
Test: manually verified that clicking on the background of the
lockscreen scene doesn't bring up the bouncer anymore. Tested with
flexiglass on as well as off.
Flag: NONE this is a very simple change and the non-flexi codepath isn't
flag protected

Change-Id: I2ce55660a14622b46b1bdc524c42817b0d7b4521