Refresh the preferred APN after "Reset to default"

Previously, ApnSettings will updated the preferred APN selection when
data connection changes, but this is not the source of truth.

Observe the preferred APN directly to fix.

Fix: 257316932
Test: manual - on ApnSettings
Test: unit test
Change-Id: Ie323316ab8f7fa63edf5cf90633bcdd4486728c4