AudioService: modify stream delay for touch exploration

When accessibility services are running, in particular touch
 exploration, the default stream override delay in AudioService
 makes it hard for the user to change the notification volume
 as when TalkBack speaks, the user would control the media
 volume during the utterance, and up until 5s after its end.
Use a shorter delay when touch exploration is enabled.

Bug 17140435

Change-Id: Iabadb9778f2957b5aa0aebd1599f2d69bd83222b