[PT11] Move handleDeprecatedGlobalHttpProxy to ProxyTracker

This contains a significant logic change : it will load the
deprecated proxy settings synchronously instead of on the next
run loop. I think this is okay because it would happen almost
immediately anyway, and there is nothing in ConnectivityService
that might be changing this setting in the mean time. As for
the possibility that this was executed in the handler because
of possible disk access, I want to point out that the
loadGlobalProxy method that now calls this was already doing
those same similar accesses.

Test: runtest
Change-Id: Idc6f260e2a337689dc274eb758eb00f6a31089bb