NetworkScoreManager: Add @SystemApi for wifi mainline module

Add formal API's for API's that the wifi mainline module uses:
a) NetworkScoreManager.requestNetworkScores - Request new network scores.
b) NetworkScoreManager.registerNetworkScoreCallback - @SystemApi wrapper
over the existing registerNetworkScoreCache.
c) NetworkKey.createFromScanResult - Create NetworkKey from ScanResult.

Also,
a) Converted the existing WifiNetworkScoreCache callback implementation to
support both the new @SystemApi (used by wifi) and the old @hide API
(used by settings).
b) Stopped invoking dump on all the callbacks from NetworkScoreService.
The dump of each callback should be invoked at their client site (i.e
wifi service should dump state of the callback it registers with the
service).
c) Added a helper method |dumpWithLatestScanResults| to help dump the
state of the WifiNetworkScoreCache from wifi service.

Bug: 144487252
Test: a) Device boots up and connects to wifi network.
b) Manually verified that network scores are being requested & updated
with the new interface.

Change-Id: Id5a66189150e7a088127519373a832f63bdd12ac