VT: Refine codes in AAVCAssembler.

 - Fixed a crash problem by overflow when rtp time hits 2^31
 - Code clean up for jitter buffer's logging.

Bug: 165061754
Change-Id: I1c9f39d8857870e1590d1f320f0385e0a500c7e6
Signed-off-by: Kim Sungyeon <sy85.kim@samsung.com>