Sleep 1s before force stop and use killBackgroundApp

Leave a bit of time before force stopping the application. This might
help prevent bad numbers.

Call killbackgroundApp after doing force-stop to make sure nothing is
still running.

Bug: 117094510
Bug: 117888316
Test: atest google/perf/app-startup/third-party-apps/cold-dropcache-stable-test
Change-Id: I47792f99b38edf2c40f61cc3f6817d82e6339d12