Retire AsyncTask in BuiltInWallpaperAsset

- Replace DecodeBitmapAsyncTask with Executors
- Replace DecodeBitmapRegionAsyncTask with Executors
- Replace DecodeDimensionsAsyncTask with Executors

Bug: 198711500
Test: manual
Change-Id: I8fad4a5b9c2f038e4366d94b828ee84978d90c4a
1 file changed