[WIP] Support for ServoSession - take 2

[paulrouget]

This is an alternative approach to #498

In this case, we do not extend GeckoSession but introduce a wrapper around all the things that need to be abstracted. This is still WIP (Servo part not done yet). But I would like to get some early feedback before finishing this.

Pros/Cons for the 2 approach:

take 1 - #498

Pros: lot less boilerplate code. Less intrusive
Cons: Will break easily with any GV update (like: https://bugzilla.mozilla.org/show_bug.cgi?id=1492771)

take 2

Pros: more solid to changes in GV
Cons: rather intrusive and requires quite a lot of boilerplate code.

@MortimerGoro What do you think?

[MortimerGoro]

I'd use the #498 approach as the first step. I see that as an easier & non intrusive solution until Android Component API is ready to use. IMO the efforts of doing the perfect abstraction should be part of the android components.

I think #498 will be ok for GV updates. We only need to override the public API methods we are using, and nothing to do with protected/private methods or LayerSession, @snorp correct me if I'm wrong.

Regarding the code in this PR, I'd try to avoid the isGecko() kind of methods, ideally the interface should be totally agnostic.