[Testing] Renamed incorrectly named FloatingHeaderViewTest

FloatingHeaderviewTest was named with ...Tests (plural), preventing it from
being excluded from launcher-testing-helpers-robo and getting included
in some unintended tests.

Bug: b/341795751
Flag: EXEMPT test renaming.
Test: Running FloatingHeaderViewTest
Change-Id: I8149f0a5548e77674bb2a23a34bd7671e0a3b371