hle/service: Replace global system instance calls with instance-based ones

Migrates the HLE service code off the use of directly accessing the
global system instance where trivially able to do so.

This removes all usages of Core::CurrentProcess from the service code,
only 8 occurrences of this function exist elsewhere. There's still quite
a bit of "System::GetInstance()" being used, however this was able to
replace a few instances.