Serially (un/)register score cache in WifiTracker.

Before the cache unregistration logic was posted to the worker thread.
However, when WifiSettings is quit, the onDestroy method immediately
kills the WorkHandler. This caused unregistration to never occur, which
left the cache registered and the listener to try to post work on the
dead worker thread. The cache is now registered and unregistered
serially in start and stop tracking, respectively. Furthermore, the
cachelistener is running on the mainHandler as it is a lightweight task
(posting to work handler).

Bug: 34891717
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/WifiTrackerTest.java

Change-Id: I7739fdbe036be5b2870be45f33f670fd69d69af1