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Improve the performance when loading image wallpaper colors
When SystemUI start up, ColorExtractor will get wallpaper colors so status bar can use it to update theme, however, this will also block SystemUI start up process if the wallpaper is image wallpaper. In order to break the latency, we can extract getWallpaperColors method from ColorExtractor's contructor, and do it at another thread, so it won't block SystemUI start up. Bug: 124279857 Test: atest ColorExtractorTest Test: Manual test change theme feature can work normally. Change-Id: I8589b28a17b0a2b33b4bbbda2265080af983f26a