Fix broken tests

1) Remove AssistantFeedbackPreferenceController since it didn't use.
2) Refine the AssistantFeedbackPreferenceControllerTest, we need to use
vpn manager object now. (See ag/13519006)

Test: Run robo test
Fix: 180576544
Change-Id: I719711d83201d571427b9f46a6026e0831cddab6
4 files changed