Fixes ScreenUndimDetectorTest

Do not call setUp() multiple times during the test. setUp() method will
create a new instance of the detector every time. Instead, we should
just reset the detector and reuse it.

Test: atest ScreenUndimDetectorTest
Bug: 284387743

Change-Id: Iaa28a5ab6c936ffe5de23a193d1883c621ae9c65