Fix VCN crash when retrieving carrier config while phone is gone

This CL calls CarrierConfigManager.getCarrierConfigSubset instead of
getConfigForSubId to guard against crashing when phone process has gone.

The former method is designed for system components to retrieve carrier
configs with consideration of phone process may be gone (e.g. during
rebooting from previous crash).

Bug: 319791612
Test: atest TelephonySubscriptionTrackerTest
Change-Id: I784610b243db2a666c1fe89b1438950a2f6a1478