A11y: Annotate the QS brightness slider properly.

Instead of just clobbering SeekBar's awesome feedback with
"Display brightness" right on the slider, we move that
string up to the parent and set it as the slider's label.

The new utterance will be: "<standard seekbar feedback> for
<label>", which in this case comes out as:

"Seek control, fifteen percent, for display brightness."

Bug: 20343730
Change-Id: I1223b3b07b6bf67ee3df22094ec5976b5188d968