Be explicit about which clearWallpaper to use

With @SystemApi included in test_current, there are two clearWallpaper
visible at compile time. Explicitly call the no-arg one.

Bug: 129133308
Test: m CtsWindowManagerDeviceTestCases
Change-Id: I1c85794a695aba9d9f5b3df08c212a302036982b
1 file changed