Limit item count in QS detail

- Introduce a maximum number of items to be shown in QS detail view
  depending on the device configuration.
- For Wifi and Cast, make the panel exactly that height as we would
  show the maximum count of items, in order to prevent that the
  buttons jump around.

Bug: 18665311
Change-Id: I7c0c383a902d295f1b486b8a074e15b990fbe562