libstagefright: fix issues in AH263Assembler

- if the received rtp packet belongs to the current
  candidate access unit but arrived later, this packet
  still needs to be inserted into this access unit before
  submitting it. Otherwise this incomplete unit would be
  sent to the decoder and cause some potential errors.

- move the functionality for checking the malformed packet
  and calculating the offset of the payload header to one
  function.

Crs-Fixed: 694476
Change-Id: Ie66524ac4b130c443c12bfb007593e5eb4a24019