Settings: Add summary for launch music app when headset is connected

This commit adds a summary for the 'launch music app when headset is
connected' feature. I made this change because in some languages
(e.g. German) the title to descripe this feature got to long for
one line. That just looked aweful.

Before: http://goo.gl/TNDX7
After:  http://goo.gl/PYVme

Change-Id: I67d74e4df5824a8bfaf13dbe708cdef87fbe738c