[WifiTrackerLib] Fix Saved Networks page showing "Saved by <package>"

Fix Saved Networks mistakenly showing "Saved by <package>" for networks
that do not have a valid app label to display.

Bug: 156324411
Test: manual visual verification that the mistaken string does not
appear in Saved Networks page

Change-Id: Ife31f095bc318edc92be6b84575cdc938e185807
3 files changed