Change groups to use number in the expander.

* Old codepath is maintained; switch with config_showNotificationGroupCountInExpander.
* Use the default expander colors for low priority header.
* Uses the correct color for the old group overflow.
* Fixes the header view to correctly accomodates growing expander.

Bug: 181048615
Test: manual, visual inspection of groups and notification types that use headers
Change-Id: Ia314a6028645016df39d3e948fdc4eb194ed9ff2