Move SettingObserver#setListening out of frequent path

FooterActionsController#setListening is called in the main thread
because it updates views. There was a register/unregister content
observer in that path that was causing jank.

As the setting doesn't change frequently, move the register/unregister
to match the view attached/detached.

Test: atest FooterActionsController
Test: manual
Fixes: 220874628
Change-Id: I9b5130eaee71ab8d5eb226a3078631ac7d153e23