grpc-proxy: Start the gRPC server and waiting thread much sooner

grpc-proxy: Start the gRPC server and waiting thread much sooner

This change moves a bit the start-up sequence of the gRPC server and
when it connects to the compositor.

Changing the start-up sequence avoids waiting for the channel to change
its state from disconnected to connected, and only wait for the wayland
connection to take place. Otherwise, we would wait first for the wayland
connection to take place, then wait for the gRPC server to start up
and finally wait for the channel to be in connected state, all which
would incur a massive waiting time.

Moving it a bit early requires to at least verify that we have the proxy
side (the wayland connection) is already set-up at that time.

Bug-AGL: SPEC-4912
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Ied88a917df8ff98fefa601103ce04e13c8bb21ac