Ensure all flicker tests have an @Test annotation

If a test method is missing the @Test annotation the behavior seems inconsistent. E.g., when i run:

```
atest FlickerTestsActivityEmbedding:com.android.server.wm.flicker.activityembedding.splitscreen.EnterSystemSplitTest -- --module-arg FlickerTestsActivityEmbedding:include-annotation:androidx.test.filters.FlakyTest
```

Junit can't find the test (correct)

However, when i run:

```
atest FlickerTestsActivityEmbedding:com.android.server.wm.flicker.activityembedding.splitscreen.EnterSystemSplitTest -- --module-arg FlickerTestsActivityEmbedding:exclude-annotation:androidx.test.filters.FlakyTest
```

Flag: TEST_ONLY
Bug: 342596801
Test: atest WMShellFlickerTests FlickerTests
Change-Id: I4d48fd9e624204b5730f05bbe2970fb9781d1559