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.
This works with the current code base. Selendroid does not use the new process name scheme (https://github.com/appium/appium/blob/master/lib/devices/android/selendroid.js#L416)