[WebView] Listen to window insets change

Overriding these methods in preparation for notch and safe area implementation
for WebView.

A corresponding implementation for Chrome app does this:
https://source.chromium.org/chromium/chromium/src/+/master:components/browser_ui/widget/android/java/src/org/chromium/components/browser_ui/widget/InsetObserverView.java;l=179;drc=b17e62321db0f69a9131d3bb87ab9cef502a0eaf?originalUrl=https:%2F%2Fcs.chromium.org%2F

An alternative would be to call View#setOnApplyWindowInsetsListener(),
but that might not be as straightforward and might conflict with app's own
usage around windowinsets.

Returning null from a default implementation and use super method until
we can actually implement it in the chromium repo.

(Note: I would like to merge this to R if possible.)

Bug: 159610496
Test: CL does not break build/test.
Change-Id: Ic0421d7347e8091e02af6c9d5a24fdb334f37c47