weblayer: Fix shell fullscreen

The shell unsets the FullscreenCallback when the activity is destroyed,
to avoid leaks, and also to avoid callbacks hitting null because the
activity no longer exists.

However unsetting FullscreenCallback has the side effect of exiting
fullscreen, which is not desired. Instead, create a
static FullscreenCallbackImpl that can unset its reference to the
Activity when it's destroyed.

Bug: 1159529
Change-Id: Ide0d440ada7d32c806b0c715538624e310e4fd8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2601667


Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840229}