Fix full widgets picker fast scroller height.

1. Fix the app title to one line.
2. Use the measured height of a header view to estimate the available
   fast scroll bar height.

Test: Open full widgets picker. Scroll to the bottom by scroll gesture.
      Observe the fast scroll bar indicator is correctly shown.
      Scroll back to top and then use the fast scroll bar to scroll to
      the end of the page. Observe the last app row is aligned to the
      bottom of the page.
Bug: 188914448
Change-Id: I57a2419d1fbfc8f946a932eebfefb67ae0c07eb6