Background Updates: Record plugin & theme update statistics like we do for core updates.

Pass plugin/theme update objects into the Background updater for consistency with core & translations.

Merges [27905] to the 3.8 branch.

props dd32.
fixes #27633.


git-svn-id: https://develop.svn.wordpress.org/branches/3.8@27923 602fd350-edb4-49c9-b593-d223f7449a82