App smoke test: Use Dpad to poke app on TV

The swipe action on leanback launcher that doesn't
support swipe gesture may cause unnecessary focus change
and test to fail. Use the dpad key to poke the app instead.

Bug: 37234147
Bug: 37301391
Test: am instrument -w -r android.test.appsmoke/android.support.test.runner.AndroidJUnitRunner
Change-Id: I85836ee1835a4e1e55005e3e2f2ade081aa49676