[Android][3PPWM] TabImpl switches Autofill context on window changes

Before this CL, the Tab would only initially provide a context, whether
the window is attached or not. This is a problem for the autofill
service which relies on an up-to-date activity context and doesn't work
well with an application context.

With this CL, the tab updates the AutofillProvider when the context
changes.

ProductionSupportedFlagList: Not needed, no effect on WebView.
Feature: AndroidAutofillUpdateContextForWebContents
Bug: 432447902
Change-Id: I866d7f6f6641730003abe67dd7a07a0517fbd51f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6818984
Reviewed-by: Sayed Elabady <elabadysayed@chromium.org>
Commit-Queue: Friedrich Hauser <friedrichh@chromium.org>
Reviewed-by: Peter Conn <peconn@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1500744}