Add API definition for WebSettings.allowMixedContent

Introduces an API for configuring how the WebView behaves with
regard to referencing insecure content from a secure origin.

By default, apps targeting <= KK will allow mixed content. New apps
will block all insecure content.

Bug: 13948531
Change-Id: Ie773ee144e223f78b6449da0a8564192dd9c1c5d