WindowManager: Reapply magnification spec when adding windows.

Previously the magnification spec was effectively reapplied on each
frame when preparing surfaces. Following the migration to the hierarchy
it was thought this wasn't needed since parent hierarchy layers
would be magnified and Surfaces would receive them as they came and went.
It didn't end up quite so hunky-dory. While the app layer is magnified in whole,
the non app layer relies on magnification of individual windows to avoid magnifying
the navigation bar. Simply put this means we need to reapply magnification
when adding windows in addition to when the magnification spec changes.

Bug: 74221620
Test: Manual
Change-Id: Ifd17ecc837a9aa611ddc29e87aa95d2854b2af8e