Move some video codec constants to separate file.

kMaxSimulcastStreams, kMaxSpatialLayers and kMaxTemporalStreams don't
really beling on VideoBitrateAllocation.
common_types.h is going away and it feels dubious to requrie include
of the full VideoEncoder api to use them. Therefore moving them into a
seprate file/target.

Also includes some remaining cleanup of includes.

Bug: webrtc:9271
Change-Id: I7ded3d97a9a835ac756159700774445a2b93a697
Reviewed-on: https://webrtc-review.googlesource.com/c/117305
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26299}
diff --git a/video/BUILD.gn b/video/BUILD.gn
index 50a106c..f82668a 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -499,6 +499,7 @@
       "../api/test/video:function_video_factory",
       "../api/video:builtin_video_bitrate_allocator_factory",
       "../api/video:encoded_image",
+      "../api/video:video_bitrate_allocation",
       "../api/video:video_frame",
       "../api/video:video_frame_i420",
       "../api/video_codecs:create_vp8_temporal_layers",