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