SDK / exoplayer

The new code in Libgav1GetFrameBuffer is copied from
libgav1/src/frame_buffer_callback_adaptor.cc. It may become libgav1
utility functions available to libgav1 clients in the future.

The Libgav1FrameBuffer struct in the old frame buffer callback API is
defined as follows:

  typedef struct Libgav1FrameBuffer {
    uint8_t* data[3];
    size_t size[3];
    void* private_data;
  } Libgav1FrameBuffer;

Copy these three fields to the JniFrameBuffer class as private data
members and add the RawBuffer() and Id() getter methods.

The existing AlignTo16 function is replaced by the copied Align template
function.

PiperOrigin-RevId: 293709205