Enable parameterized tests in DesktopRepositoryTest

Adds missing call to setFlagsParameterization to allow tests to run with all flag combinations.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 362720497
Test: atest DesktopRepositoryTest
Change-Id: I27edcc1f24fb6564dce8bfc8c4d734ca4a9c457c