Annotate desktopmode flicker tests as postsubmit

The legacy APT configs executed only tests annotated with `Presubmit` in presubmit

Now we run everything not annotated with `Postsubmit` or `Flaky` in presubmit. This accidentally promoted these tests to presubmit

Bug: 311643481
Test: WMFlickerTestsDesktopMode
Flag: TEST_ONLY
Change-Id: Ifcae4e0e9edcfb8bf748cebb46cc3823dd835264