SystemUI: Calculate paged QS tiles height properly

When QS is re-inflated during UI mode change and we're on the
3rd or higher QS page, the first QS page is misaligned and
hence height returns 0, resulting in footer and media panel
position overlapping the QS panel. Return the maximum height
among all present QS pages to fix this issue.

Change-Id: I539babdb75c114cc44b4213ff114d4272be22ef6