HeavyWeightSwitcher: Use selectableItemBackground

Change the list item background from list_selector_background to
selectableItemBackground.
Also changes some paddings, so that list item is padded while keeping
icon aligned with the dialog description text.

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: Ib4cdfcb16ccce25aa4a73cfb583da5603bef14c4