grpc-proxy: Add set_app_fullscreen functionality

grpc-proxy: Add set_app_fullscreen functionality

This implements set_app_fullscreen which clients can set-up before being
mapped.

The worthwhile change here was the fact that transitioning between
fullscreen, normal, and float would cause invalid tracking of the active
window when switching between these states. This would make floating
operation display the incorrect active window, so in order to reconcile
that, we only update the previous surface if it is different that the
current active one. Otherwise this fairly similar to set_app_float.

Bug-AGL: SPEC-4673
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Ie912c86ff7ac38d034cf4d97b2adbc5ef47ce9d3