Generate fallback speed label in AccessPoint.java

Cache all seen matching ScoredNetworks, and use these networks to
generate a fallback curve. This cache is kept in sync with WifiTracker's
WifiNetworkScoreCache, hence it does not permanently create any new
objects other than the map itself.

Bug: 63073866
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/AccessPointTest.java

Change-Id: I0cdf17f63f0238e0e3a8324190f75d82beaa6672