New AudioTrack C++ API for audio timestamps

This new API is intended to replace latency(), especially for A/V sync.
The application will receive periodic timestamp notifications.  The period
is unspecified, but will likely be more frequent after a pause or stop,
set position, underrun, display on/off change, route change, or when audio
framework notices drift.  It will be up to the higher level application
(e.g.  Stagefright) to reconstruct a clock that updates more frequently.

The current latency() method doesn't indicate when latency changes
due to screen on/off state, route changes, etc.

Includes squahsed change-Id: I2082f8752040be0c234b1a6f1be2e269abf2ce7c
 Dummy implementation of AudioTrack:getTimestamp()
 Rename AudioTrack::Timestamp to AudioTimestamp.
 Renaming and pulling up to a higher level allows more modules to use it.

Change-Id: Ibf7f6a207c3f8d8697f25ede2cd5200697fadb86
(cherry picked from commit dd69eb893867634fd169c03204a6ad7c74b351e7)