OboeTester should stop test from onPause instead of onStop if background operation is not checked

If background operation is not selected, OboeTester should stop its test when moving to background. This happens when `onPause` is called. When `onStop` is called, it is indicating the activity is no longer visible. See activity lifecycle at https://developer.android.com/guide/components/activities/activity-lifecycle#alc

With current implementation, the test will still be running such as playing output for slightly longer with using gesture swiping up to move the app to background.