AccessPoint - Add Builder; Add unit tests for compareTo()

Add a Builder class to make testing easy and clean.
Add unit tests to enforce the sorting order that we want.
Refactor some compareTo() internals to use existing methods rather than
duplicating functionality.
Refactor isReachable() functionality to a new method.
Update tests to use Truth assertion library.

Test: Run local AccessPoint unit tests:

adb shell am instrument -w -e class
com.android.settingslib.wifi.AccessPointTest
com.android.settingslib/android.support.test.runner.AndroidJUnitRunner

Bug: 35042429
Change-Id: I7c8a1ca69800f62e1fb13141e58b4e52b2bc3d70