[Android] Add null checks to the PlusAddressCreationBridge.

Both WindowAndroid and TabModel can be null when retrieved from the
WebContents. This CL adds necessary null checks on the WindowAndroid,
TabMode, corresponding Java objects and the Activity retrieved from the
WindowAndroid.

Bug: 364181540
Change-Id: I3ea55a55bc0f206a684877a30baef8444536e47c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5831440
Commit-Queue: Timofey Chudakov <tchudakov@google.com>
Reviewed-by: Friedrich Hauser <fhorschig@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1350721}