Copy WifiNetworkScoreCache into frameworks/base. Add tests.

Begin process of migrating WifiNetworkScoreCache.java from System Server
(//frameworks/opt/net/wifi/service/java/com/android/server/wifi/) to a
SystemApi. This CL is a straight copy and paste of
WifiNetworkScoreCache.java. Only new code is located in
WifiNetworkScoreCacheTest.java

Bug: b/33050254
Test: Unit tests added to cover most common use cases.
Change-Id: I7510420d293339ce7811fcf71a6efceb8d809c3a