Vp9 flexible mode fixes

- Enable vp9 flexible mode in VideoEngine if 3 spatial layers are set.
- Enable flexible mode in loopback tools and quality tests.
- Reset first active spatial layer on keyframe in encoder.
- Ensure duplicate references are not set by the sender in video header.
- Set references manually for flexible mode in vp9 encoder.
- Delay new activated layers until next base layer frame.
- On receive side put each spatial layer as a separate frame to FrameBuffer
  and return several frames combined from FrameBuffer.

Bug: webrtc:10049,webrtc:9794,webrtc:9784
Change-Id: I01e69f134cc145deba666ccc92deb1d37a324ede
Reviewed-on: https://webrtc-review.googlesource.com/c/112289
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25895}

modules/video_coding/codecs/test/videocodec_test_libvpx.cc

@@ -124,9 +124,9 @@ TEST(VideoCodecTestLibvpx, ChangeBitrateVP9) {
       {500, 30, kNumFramesLong}};
 
   std::vector<RateControlThresholds> rc_thresholds = {
-      {5, 1, 0, 1, 0.5, 0.1, 0, 1},
-      {15, 2, 0, 1, 0.5, 0.1, 0, 0},
-      {10, 1, 0, 1, 0.5, 0.1, 0, 0}};
+      {5, 2, 0, 1, 0.5, 0.1, 0, 1},
+      {15, 3, 0, 1, 0.5, 0.1, 0, 0},
+      {10, 2, 0, 1, 0.5, 0.1, 0, 0}};
 
   std::vector<QualityThresholds> quality_thresholds = {
       {34, 33, 0.90, 0.88}, {38, 35, 0.95, 0.91}, {35, 34, 0.93, 0.90}};