Have HWUI validate the WebViewFunctors at registration

Also tweak how webview's plat_support creates the functor
sruct to be thread-safe, avoids any potential race conditions
even though WebView itself is stated to be thread-hostile in
general. It's too easy to have this just be defined-safe instead.

Bug: 186814981
Test: build & boot, no crashes in real-webview apps
Change-Id: I06f02a279e248fee375ce133c5ce9a2250665ad9