Add autoWebview capability

Milestone 1.2 issue #2715

Allow user to enter directly into webview context. Adds universal capability `autoWebview` (`true`/`false`) and Android capability `autoWebviewTimeout` (in seconds, defaults to `2`), to set hot long to wait before checking for the webview. 

Also fixes Android/Selendroid webview tests.
instead of an external resource, I'd like for these tests to hit the webpages which the appium server itself hosts for testing purposes. Check out the other web tests to see how this works. This will reduce flakiness.