SurfaceTexture: allow set_buffer_count(2)

This change relaxes an error check in SurfaceTexture::setBufferCount to
allow clients to explicitly set a buffer count of 2.  The clients that
will do this are camera and video decode.  Previously it was thought
that for those clients we would always use async mode, which requires a
minimum of 3 buffers.  However, we now believe that for some devices it
may make sense to use synchronous mode (with 2 buffers) to reduce memory
usage.

Bug: 5088418
Change-Id: I620a0ef75075745be9d6c8219e0246aaf33ba950