Set stateDescription during ProgressBar initialization

onProgressRefresh() is only called when the progress changes. So we should set
stateDescription during initialization.

Test: tested with talkback test app. Without this change, the
stateDescription is not available when we focus on the seekbar. After
the change, the stateDescription is avaiable.

Change-Id: I56071ed28e6917c6bd14162884b10440c6955751