Reland "Moved BitrateConfig out of Call::Config."

This is a reland of 5897fe27abcbe70f706cc23adc26147e0581f97e.

Adding back CallConfig::kDefaultStartBitrateBps as deprecated.
Also making BitrateContraints::kDefaultStartBitrateBps private to stop
it from being used in other places.

Original change's description:
> Moved BitrateConfig out of Call::Config.
>
> This prepares for a CL extracting the bitrate configuration logic from
> the Call class.
>
> Also renaming BitrateConfig to BitrateConstraints.
>
> Bug: webrtc:8415
> Change-Id: I7e472683034c57bdc8093cdf5e78e477d1732480
> Reviewed-on: https://webrtc-review.googlesource.com/54400
> Commit-Queue: Sebastian Jansson <srte@webrtc.org>
> Reviewed-by: Stefan Holmer <stefan@webrtc.org>
> Reviewed-by: Niels Moller <nisse@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#22104}

Bug: webrtc:8415
Change-Id: Iacfe2d6daedff710832ab89210c7c66d4403c93b
Reviewed-on: https://webrtc-review.googlesource.com/55980
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22123}
diff --git a/pc/peerconnection.cc b/pc/peerconnection.cc
index 7191a28..14cff94 100644
--- a/pc/peerconnection.cc
+++ b/pc/peerconnection.cc
@@ -2878,7 +2878,7 @@
     }
   }
 
-  Call::Config::BitrateConfigMask mask;
+  BitrateConstraintsMask mask;
   mask.min_bitrate_bps = bitrate.min_bitrate_bps;
   mask.start_bitrate_bps = bitrate.current_bitrate_bps;
   mask.max_bitrate_bps = bitrate.max_bitrate_bps;