Don't use a high-priority notification on auto-join.

If the device connects to a network automatically and not through
user action, a high-priority notification is intrusive and is
inconsistent with other networking notifications, which are
usually only high priority if the network is manually selected.

Bug: 130766237
Test: see next CL in patch series
Change-Id: I8824f2d1a0efeb6cb75e430ef5159ebce0018779