Fix AccessPointPreferenceTest and add updateIcon test.

Dependency injects the getIcon() functionality in AccessPointPreference
to enable mocking the method in robolectric where it normally fails to
find the resource. Adds a test to verify that getIcon is called on
refresh. Also modifies the constructors to reduce redundancy.

Bug: 35854035, 64898200
Test: make -j40 RunSettingsLibRoboTests
Change-Id: Iaf9e27651db1d9f16f58ac80ac413e9f84f08b39