[flexiglass] BrightnessMirrorViewModel is a SysUiViewModel

As per go/sysui-arch:summer-2024

Bug: 354270224
Test: unit tests still pass
Test: manually verified that the brightness mirror works in flexiglass
Flag: com.android.systemui.scene_container
Change-Id: I98cd6cb916a074507b721256e309ab69d42dab7d