Update test::CreateVideoStreams to use the configured min bitrate if set.

VideoSendStreamTest.ReconfigureBitratesSetsEncoderBitratesCorrectly:
Configure min_bitrate_bps via VideoEncoderConfig (and remove
implementations of VideoStreamFactoryInterface used to override
the default min_bitrate_bps configuration).

Bug: none
Change-Id: I488f78d8d145b24fd65dd29fdc979e543fcb0b7c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/132798
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Commit-Queue: Åsa Persson <asapersson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27711}
diff --git a/test/encoder_settings.cc b/test/encoder_settings.cc
index cc1994f..1673160 100644
--- a/test/encoder_settings.cc
+++ b/test/encoder_settings.cc
@@ -66,6 +66,10 @@
               : DefaultVideoStreamFactory::kMaxBitratePerStream[i];
       target_bitrate_bps = std::min(max_bitrate_bps, target_bitrate_bps);
 
+      if (stream.min_bitrate_bps > 0) {
+        RTC_DCHECK_LE(stream.min_bitrate_bps, target_bitrate_bps);
+        stream_settings[i].min_bitrate_bps = stream.min_bitrate_bps;
+      }
       if (stream.max_framerate > 0) {
         stream_settings[i].max_framerate = stream.max_framerate;
       }