Initial implementation of dismiss gesture + inline settings icon

- Added a view to display / control the gear behind the noti
- Updated SwipeHelper to hide / show the gear based on the gesture
- Only one notification or notification group can show the gear
  at a time

Couple of things left to do that will come in a follow up CL:
- Animation (to / from inline controls, fading gear based on movement)
- For children in a group there needs to be a grey background

Bug: 22451710
Change-Id: I2f93edf48109a82bab0839c73cbaba1535caee9a