Merge restored wifi APs, don't overwrite

We now preserve any already-known AP configuration information when
restoring wifi from backup, instead of flatly overwriting the known
definitions with the historical ones.  This means that if you are
performing setup while connected to an AP unknown in the restored
dataset, you will retain your connection instead of seeing it drop
partway through the restore process because suddenly the supplicant
"forgot" how to connect to it.

Duplicates are resolved by retaining the currently-defined network
configuration and discarding the historical one.

Bug 6443790

Change-Id: I1f44cc6a01fc4ae7c4b680682a10fcb7a0be65dc