Add Trace points to WallpaperColors

WallpaperColors processes Bitmaps which can be slow (and cause jank)
when called on the main thread. This adds trace points so we can debug
these calls as a source of jank.

Bug: 287209189

Test: ran a perfetto trace on device
Change-Id: If0cb5d3d6bf04c404fae2b1a6229cdb92946526b