Cache Feedback Enabled Setting in Controller

AssistantFeedbackController#isFeedbackEnabled was making a binder call every time.  We are caching this state in the feedback controller now and listening to changes in the setting.
This change also moves the NOTIFICATION_FEEDBACK_ENABLED setting to Settings.Global.

Bug: 161512521
Test: atest AssistantFeedbackControllerTest
Change-Id: I79526f36577de2b170b703e3cbc89a29dbb0649b