Add the SurfaceTexture C++ implementation.

This change adds the C++ implementation of SurfaceTexture and related
classes. The goal of this is for a SurfaceTexture to be passed to
camera service or Stagefright in place of a Surface to allow camera
preview or decoded video frames to be streamed to an OpenGL ES texture
that an application can use.

Change-Id: I55c83a7017f1ecb81c9c9e3252cbd118b914296c