[E2E] Fix NPE due to window inset consumer race

It's possible for the viewport-fit to change multiple times before the
registered WindowInsetsConsumer is invoked, leading to NPEs when trying
to access mSystemInsets in #adjustEdges.

Checking for null mSystemInsets before invoking #adjustEdges
in #drawToEdge and adds two regression tests to prevent future crashes.

Fixed: 329875254
Change-Id: I4b3aaaa3db65ec17ff63d8147c24657b86c14243
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5378615
Commit-Queue: Lijin Shen <lazzzis@google.com>
Auto-Submit: Theresa Sullivan <twellington@chromium.org>
Reviewed-by: Lijin Shen <lazzzis@google.com>
Cr-Commit-Position: refs/heads/main@{#1274437}