Mms: Fix Visibility of Action Bar Text

Prior to this change, the action bar that is triggered when selecting multiple
conversations for deletion had text that was to dark to read.

This: http://i.imgur.com/LmoDVBO.png --> This: http://i.imgur.com/cUTT4lb.png

Change-Id: Ifce34a1fd54e8878f9adeb53bcf53c2131ab3986