[Flexiglass] Fix brightness mirror

The brightness mirror was not properly aligned after changes for focus.

Replicate BrightnessMirrorController so things match. We also need to
make sure that the mirror container can be wider than its container in
flexiglass.

Also, have two different ways of calculating offset based on the
location of the mirror with respect to the window.

Test: atest com.android.systemui.settings.brightness.ui
Test: manual in phone, unfolded, and landscape
Fixes: 348640465
Flag: com.android.systemui.scene_container

Change-Id: Ic0c500e1dc2258e8b1cb368b8aa42b9ad183ce65