Checking that the connection service can be resolved when setting new connection manager.

This is to fix an edge case where downgrading could cause an invalid
connection manager to be set, resulting in all calls failing.

Also filtering out unresolveable phone accounts (which prevents the UI
from showing invalid choices).

Bug: 18225329
Change-Id: If46dea2937af42b81d3d7062c385f98370015cbe
2 files changed