Deprecate and stop generating tickerViews.

Tickers have been replaced in the UI by heads-up
notifications (and not creating the tickerView will save
memory, almost 150k if you have an xhdpi largeIcon).

Update docs to explain that tickers are just for
accessibility now.

Bug: 16395266
Change-Id: Ib1372b34c1c261f64228bbe705d174675cd007dc