Fix HTMLViewer rendering when edge-to-edge is enabled.

This change makes HTMLViewer account for the window insets. Without it
when edge-to-edge mode is enabled, the page contents will be drawn
behind the status bar and the action bar.

Test: manual

Flag: EXEMPT bugfix
Bug: b/330574528
Change-Id: I9cb155e852cab0965550cfb45815c89d96745e30