Webview overlay support

The basic idea is to create a child surface control from the root surface control passed from ViewRootImpl to the render thread.

Transactions are sent back to the java layer to get merged.

In case of offscreen layers, SurfaceControl must be disabled.

This new feature is disabled for Vulkan at the moment, a new CL will be used to enable the support.

Bug: 173671170
Test: manual, webview apks
Change-Id: I119405d13eca3c59fd3ec78e50dc7739f78411d4