AudioService: Define minimum levels for volume streams.

 - Set a floor of 1 for voice call + bluetooth sco, otherwise 0.
 - All api calls validated to ensure a floor of the min level.
 - Volume UI updated to shift the seekbar by the min value.
 - Remove duplicate static max method in AudioService.
 - Ensure streams with a min level > 0 are not considered muteable.

Bug: 19260237
Change-Id: I213180c9c277f51bd3897b7f777e5f88ed1db125