video_rtp_session: Fix signed integer overflow

`last_REMB_*` can be at `time_point::min()` when `delayMonitor()` is called.  This
results in a signed integer overflow, which UBASAN doesn't like.

Fix this by setting `last_REMB_*` to `clock::now()` before setting callback.

Change-Id: I0b24d759927c2eb7e23253a16333ba54dc3f9edf