Fix scrubbing behavior on keyguard music transport

This fixes a bug where the music scrub position would snap
back to a previous position. The problem was caused by latency
in the music application responding to scrub position changes.
The latency would mean that we'd get a response periodically
to some historical scrub position change.

Since we can't know when the state will become stable, we
just wait a little bit since the last update change before
continuing to update the scroll position.

In order to keep the music client from falling behind, we
throttle scrub updates.

Fixes bug 11351267

Change-Id: I6204833328751d1da781b4e078a2d557c1f93ff3