Implement score cache eviction in AccessPoint.

This prevents cached scores from being held indefinitely and used for
SSID fallback logic in WifiTracker (Picker).

Bug: 63073866
Test: runtest --path frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/AccessPointTest.java
Change-Id: Ib351d20db30dfd18b69bb1f8e4d4f26fc6b74ef0
Merged-In: Ib351d20db30dfd18b69bb1f8e4d4f26fc6b74ef0