libstagefright: Dynamically allocate memory for esds & avcC atoms

In MPEG4Extractor, buffers for the esds and avcC atoms was
statically allocated for 256 bytes.  But the esds atom can
be much larger. Hence, this change replaces the static
allocation with a dynamic allocation.

Change-Id: Ifc082ec77a2d03a6fff08ed5df2247636747c550