Beginning with Android 4.4, WebView is based on Chromium (version 30), and includes native support for WebSocket.
Older versions of Android have WebView based on older version of WebKit and (as far as I know) lack support for native WebSocket: http://jimbergman.net/webkit-version-in-android-version/
Which means: you either need to use a WebSocket Plugin for Phonegap to add WebSocket to WebView OR wait for AutobahnJS finish the HTTP longpoll based fallback (under development) - and use a WAMP v2 server that supports WAMP-over-longpoll.