AglShellGrpcClient: Add activation with gRPC proxy

This follow-ups in footsteps of flutter-homescreen to have activation
using gRPC. Note that setting up wayland surfaces is still need to
have the agl-shell protocol available. In Qt this is managed directly
by Qt/QPA while on other toolkits this happens at a lower level
(flutter-auto or chromium CEF).

With it, create a listening thread for gRRC events in order to perform
the initial start + activate sequence.

Bug-AGL: SPEC-4912
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I019d0e7944dde02c84b2841e22d3971f226d610a