Context for 'Installed' and 'Already Installed' theme strings.

props SergeyBiryukov.
see #27889, for trunk.

Built from https://develop.svn.wordpress.org/trunk@28159


git-svn-id: http://core.svn.wordpress.org/trunk@27990 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Use correct echoing context function. see #27889.

Built from https://develop.svn.wordpress.org/trunk@28160


git-svn-id: http://core.svn.wordpress.org/trunk@27991 1a063a9b-81f0-0310-95a4-ce76da25c4cd