Add unit test for WallpaperService.Engine#getRenderingSurfaceControl

Adds a unit test for the `getRenderingSurfaceControl` method to ensure it returns the expected initial value.


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Original Change: ag/35063715
Test: passed ABTD tests http://go/forrest-run/L57400030017258632
Bug: 431235865
Flag: TEST_ONLY

Change-Id: Iad99426f516cc7d0d2d899e363126872eb0354b3