Decouple OverviewProxyService from notifications

Notifications doesn't talk to Launcher through it and doesn't need to be
responsible for registering that communication channel.

Test: NotificationLockscreenUserManagerTest
Test: OverviewProxyServiceTest
Test: flash device, open recents
Test: will monitor car_system_ui_tests flakiness
Fixes: 319732207
Change-Id: Iefcbf83199c47261ebb1bc2119bfb290f990bf2f