Re-enable mirror for brightness slider

There's a current issue (both in old and new brightness) with a scrim
covering the background when sliding in lockscreen (but not with device
unlocked).

Bug: 141690457
Bug: 171319433
Test: manual
Test: atest com.android.systemui.settings.brightness
Change-Id: I904af84a574f1ad1bfff3d014c45fe6b94094224