Protect volumepanel slider creation from race condition.

It is possible for 2 different threads to poke the mStreamControls at the same time,
causing the monkey bug mentioned in the bug report below.

Bug: 6411852

Couldn't think of any other reason Stream type 3 (MUSIC) wouldn't exist in the list.
It's possible that a programmatic call came in at the same time as the volume key press.

Synchronizing blocks where the mStreamControls are populated and accessed.

Change-Id: Ifedec6b0f8bad9634cb9e079fda785c433bdb7a7