[Adaptation] VideoStreamAdapter broadcasts adaptations

This moves this responsibility from the ResourceAdaptaitonProcessor
to the VideoStreamAdapter. A new interface for listening to adaptation
changes was added, and the ResourceAdaptationProcessor now listens on
the VideoStreamAdapter for those changes.

This means that going forward,

1. We can do adaptations outside of resource limitations, like setting
prior adaptations on a resource like initial frame dropper is designed
to.
2. Adaptations can be on a different queue than the
ResourceAdaptaitonProcessor's queue since updates are pushed through
the listener.


Bug: webrtc:11700
Change-Id: I6de0dec748dba095e702f0b9893c5afa50b51aa9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/176859
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Evan Shrubsole <eshr@google.com>
Cr-Commit-Position: refs/heads/master@{#31615}

video/video_stream_encoder_unittest.cc

@@ -205,7 +205,8 @@ class FakeVideoSourceRestrictionsListener
   void OnVideoSourceRestrictionsUpdated(
       VideoSourceRestrictions restrictions,
       const VideoAdaptationCounters& adaptation_counters,
-      rtc::scoped_refptr<Resource> reason) override {
+      rtc::scoped_refptr<Resource> reason,
+      const VideoSourceRestrictions& unfiltered_restrictions) override {
     was_restrictions_updated_ = true;
     restrictions_updated_event_.Set();
   }