Refactor ZygoteInit to support a WebView-specific zygote.

This is a non-functional change that separates out functionality
that should be shared between the system zygote and the WebView
zygote from that which is system zygote specific.

* Move MethodAndArgsCaller to Zygote.
* Split out server socket functions into ZygoteServer.
* Add a new (stub, for now) WebViewZygoteInit class.

Bug: 22084679
Bug: 21643067
Change-Id: I4c508a42af7ab7b53d10570ad53b846df7782cc4