SystemUI: avoid double ripple in QS detail view

 * Remove the style attribute of the imageview
   in order to remove background ripple for this
   view that was displayed in addition to the
   ripple of the whole layout.

Test: visual observation
Signed-off-by: Timi Rautamäki <timi.rautamaki@gmail.com>
Change-Id: I964a86bb1f4ff1539630f8f5e8fcec67e55a8a11