Make private browsing always throw

We were throwing an exception when private browsing is enabled on target
SDK versions >= K. The WebView ctor overload that accepts the private
browsing boolean has been deprecated a long while ago and the doc says:
"Private browsing is no longer supported directly via WebView and will
be removed in a future release." Moreover, torne@ mentioned that private
browsing was never really used by apps.

Now, it will throw an exception in all cases irrespective of the target
SDK version.

Bug: b:441953305
Change-Id: Ie47e1fe0f3a2fa801096388c003477370712a07c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6899533
Reviewed-by: Richard (Torne) Coles <torne@chromium.org>
Commit-Queue: Abhijith Nair <abhijithnair@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1510214}