Move some system services to their own sub package.

As a next step they can be moved into separate directories to be
built as separate modules that may or may not be included in a
particular configuration.

Moves AppWidgetService, BackupManagerService, ClipboardService, DevicePolicyMS,
and WallpaperMS.

Change-Id: I33e48d070f38186819d3a1d7a4dc3984b75a9eda