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.
:+1: if it works. I see `listWebviews` in that link and that's what I updated to use [process names](https://github.com/appium/appium/blob/668e464c5c5d844ca07d69529c0b0bde573d655f/lib/devices/android/android-hybrid.js#L42).