[flexiglass] Account for navbarheight in minVisibleScrimHeight

Footer is only visible when there is enough height in the scrim excluding the nav bar to display the stack - minVisibleScrimHeight should reflect that.

Bug: 359956998
Test: manually verified that scrim can now expand when there are just enough notifications that the footer is not visible when at rest
Flag: com.android.systemui.scene_container
Change-Id: If99f395d50e68b867b7ac2152cc0eedd40f2efde