Themes: Fix bad idmaps for system themes

For a system installed theme or a user who cleared his resource-cache,
there were empty idmaps for some packages.

[Details]
During startup PM could reach the theme package first before the target app was scanned in yet. This
would cause aapt to be skipped. Then when the target app is being installed in startup,
PM would try to create idmap pairs but it would fail since the resources from the theme
were not compiled for that target app.

Also cleaned up PM idmap/aapt creation a little bit more