Lock mSurfaceHolder=null in ImageWallpaper

There is a race condition where WallpaperEngine#onSurfaceDestroyed is
being triggered right before the image is drawn in
ImageWallpaper#drawFrameOnCanvas. Adding synchronization makes sure the
null check in drawFrameInternal avoids this.

Flag: NA
Bug: 279589555
Test: atest ImageWallpaperTest
Test: atest WallpaperLocalColorExtractorTest
Test: atest WallpaperManagerTest
Test: manual (check picker basic user journeys)

Change-Id: I650bd31faed96383258105335b5ad92448a840f3