Ensure burn-in doesn't impede insets

Calculate the min top value of eligible views in
keyguard_root_view to ensure that, when burn-in is
applied, that it doesn't enter the top inset.

Fixes: 325457611
Test: atest AodBurnInViewModelTest
Flag: ACONFIG com.android.systemui.migrate_clocks_to_blueprint
DEVELOPMENT

Change-Id: I87eb6cf2267a8463952c60eceb393237cd837b4b