Get system resources instead of phone context

Use Resources.getSystem() instead of mPhone.getContext().getResources()
to load config_cdma_home_system to resolve roaming indicator.

Fixes: 62843620
Test: runtest --path tests/ (existing unit tests pass)
Change-Id: If3af44f22f5e2b5fc090f4478ccef6192c5ecb16
Merged-In: If3af44f22f5e2b5fc090f4478ccef6192c5ecb16