[flexiglass] Fixes SysUiState related tests

SysUiState is a pub-sub-hub that preceeds Flexiglass. Multiple codepaths
write to and read state from it. Early in the Flexiglass project, a
SceneContainerPlugin was created that would act as middleman between
writers of state and SysUiState, replacing values written by legacy code
with Flexiglass-fitting ones (for example, whether the bouncer is
showing comes from the SceneInteractor in Flexiglass instead of the
keyguard code in legacy).

This plugin messes with this test. The approach I've taken is to provide
a fake plugin that defaults to overridding nothing.

Bug: 283121968
Test: N/A test only
Flag: EXEMPT test only
Change-Id: Iab6c4e3f81d1e0c0c380b815fade73d3e5bbbdc8