themes: allow system_server to relabel them dir

On a fresh install the theme service broker creates the initial
theme directory which needs to be relabeled to a themeservice_app_data_File
in order for the brokered theme service to write to this directory

RM-290

Change-Id: Ifd689a0c619c0e954192749b83a0cacaa945468f
TICKET: NIGHTLIES-3349