Use system alert theme for HeavyWeightSwitcherActivity

Instead of hardcoding the use of a light theme, use a system theme.
This allows other form factors, such as TV, to use a dark theme instead.

Test: Used HeavyWeightSwitcherActivity as test dialog.
      Tested on phone emulator to verify appearance on phone.
      Same change applied to pi-tv-dev branch tested on
      an ADT-2 to verify appearance on TV.
Bug: 133398453
Change-Id: I412c837cc316d01594c8b86a5b758cc93511458a