Support multiple windows

[sienori]

The following error occurred when I tried to duplicate BrowserView to display multiple windows: java.lang.IllegalStateException: Only one GeckoRuntime instance is allowed #25

To solve this problem, I made the following changes:

• Changed mRuntime to static to prevent multiple GeckoRuntime instances
• Changed PassStaticSurface to a non-static PassSurface to allow each instance of GeckoViewPlugin to receive a surface.

This allows the user to create multiple windows by simply duplicating the BrowserView :)

[IanPhilips]

Beautiful! I'll try out the build this weekend