Mock isDeviceEligibleForDesktopMode in WallpaperManagerServiceTests

Improve the robustness of WallpaperManagerServiceTests by mocking DesktopModeHelper#isDeviceEligibleForDesktopMode,
preventing implementation changes in that method from causing test failures.

Test: atests FrameworksMockingServicesTests:WallpaperManagerServiceTests
Flag: EXEMPTED: tests only change
Bug: 397611386
Change-Id: I3ba574ed5fa7d93de171946372382a6d86d4aa00