On Full Treble devices, servicemanager should only service
services from the platform service_contexts file.

Created new type to separate plat_ and nonplat_service_contexts,
and added new type to mapping (although I don't think this type
should have been used by vendors).

Bug: 36866029
Test: Marlin/Taimen boot
Change-Id: Ied112c64f22f8486a7415197660faa029add82d9