Fix broken robotests.

In ag/2580164 AccessPoint.java made a reference to WifiTracker, which
in term references WifiNetworkScoreCache. Since the Robolectric in the
platform still has not been updated to support API 26 (Android O), this
caused the test to break.

By definining the class in the test package, the test can be fixed until
proper Android O support is added to the robolectric prebuilts in the
platform code.

Bug: 64217019
Test: make -j40 RunSettingsLibRoboTests
Change-Id: I6cfd5c17d1aba98d43cca4828058493960a7ff79