WifiMetrics: add histogram of connectable networks

Added histograms tracking the number of networks in range of a scan.
Histograms are dimensioned on: <SSID|BSSID>*
<Total|Saved|Open|Saved_or_Open|Passpoint>

Test: Added unit test.
frameworks/opt/net/wifi/tests/wifitests/runtests.sh
Bug: 36819798
Change-Id: If7a6f2bb844394966bd2cc542dd3ebfff42609e8