Fix issue #2619247: Music sometimes stops playing when navigation talks

When a service transitions from foreground to background, we now push it
to the top of the LRU list.  Also fix the activity manager to take care
of killing processes if we go beyond a reasonable number of background
process to keep around.

Change-Id: Ic9f44c02af7a111ee6f1d06142386b301948bafe
1 file changed