[WifiNetworkSelector] Log the candidates

Log the candidates that are considered for network selection.

The logging is produced in the dumpsys output. For example:
Candidate { networkId = 5, ..., rssi = -50, ..., trusted, open }

Bug: 148217580
Test: atest WifiCandidatesTest
Test: atest com.android.server.wifi
Test: manual (adb shell dumpsys wifi | grep Candidate)
Change-Id: Ie9b0ab89a72cc8e4599aed48074498fcb0b34919
3 files changed
tree: 102b869b868a3fc884357266f4ae209e4ac3a557
  1. apex/
  2. service/
  3. PREUPLOAD.cfg