[flexiglass] Fixes quick affordance test when flexi is on

Disables the test method. Lockscreen visibility in Flexiglass isn't
controlled by setKeyguardShowing; it depends on the scene, so this logic
doesn't actually need to be tested this way.

Bug: 283121968
Test: not really tested; reproduced the failure locally with the flexi
flag enabled and slapped a disable on it due to the reasoning above
Flag: EXEMPT test change only

Change-Id: I3d4a4c04685f2e270f601d8be8f1910096fea02d