[flexiglass] Fix type inference issue in SysUiViewModelTest

In SysUiViewModelTest, not explicitly specifying the expected type runs
into a limitation where the Kotlin compiler appears to be unable to correctly infer
the return type of the rememberViewModel factory. It thinks it's Unit instead of FakeSysUiViewModel.

This change explicitly declares the type of the view model variable
to work around this issue, allowing the test to compile and run
correctly.

Bug: 434169789
Bug: 432549544
Flag: com.android.systemui.scene_container
Test: ran SysUiViewModelTest which was failing on head before this change, verifying that it now passes.
Change-Id: I58fb143088434fe9fc72438a0b8a7026305851c5