VT: Adjust an anchor time if packet comes too early.

[Problem] video played out with delay consistently.
[Cause] an anchor time set with delay if a first rtp
  packet came with delay.
[Solution] adjust anchor time if packets are coming
  too early than expected.

Bug: 183578712

Signed-off-by: Byeongjo Park <bjo.park@samsung.com>
Change-Id: I49ad1ad7e728dbb1d4ddb6f82f9a8072ad9c867d