NEW_API: Add getTimestamp() to SurfaceTexture.

Like getTransformMatrix, getTimestamp provides a timestamp for the
OpenGL texture produced by the latest call to updateTexImage().

Timestamps are measured in nanoseconds, and are monotonically
increasing.  Other properties of the timestamps (zero point,
comparability) depend on the source providing frames to the
SurfaceTexture, and need to be documented by the source.

bug:3300707

Change-Id: I380d94926d0e9f1c77bb5c1576b72d5ef4e2eba1