android_webview: add to support file uploading

This commit changes function from empty implementation
to abstract function interface in AwContentsClient for
supporting uploading file in browser. Its subclass will
override this function. In Android 4.4, browser is based
on chromium not webkit engine, chromium engine support
uploading file and will callback to client to show file
chooser, but android_webview client has empty
implementation within this callback for file chooser.

So we solve this issue by adding implementation in framework
chromium client which extends android_webview client, function
in framework webkit client and corresponding functions in
browser app.

Change-Id: I50375573848879f3c30069b1831b314f0c536163