Handle volume key events while in overview

- Normally this gets dispatched to an app's phone window to notify
  the media session manager, but because the recents input consumer
  routes to the launcher window bypassing phone window, the event
  never gets processed.

Fixes: 185520916
Test: Open overview from an app, ensure volume dialog still shows
Change-Id: I8784a6211e56f320b8b9d688fa0568c583652725