Reduce max time between repeat frame to avoid flakiness.

The frame cadance adapter injects repeat frames with a ca 1s interval
even if no new content has arrived, and the libaom AV1 encoder skips
higher temporal layer repeat frames for up to 1s. The causes a
potential race between the two that may in some edge cases result in
an extra keyframe being generated. Reducing the encoder interval to
500ms avoids that condition.

Bug: b/519058486
Change-Id: I3f84be1dbc5a2c160ce76afaa6034332f5429e10
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/485120
Auto-Submit: Erik Språng <sprang@webrtc.org>
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48078}