Move MediaConstraintsInterface to sdk/, and make it a concrete class

Bug: webrtc:9239
Change-Id: I545ebf59b078dd94bc466886616dd374e4b2e226
Reviewed-on: https://webrtc-review.googlesource.com/c/122502
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26682}
diff --git a/BUILD.gn b/BUILD.gn
index a62bb02..0f68aac 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -496,6 +496,7 @@
       "rtc_base:sigslot_unittest",
       "rtc_base:weak_ptr_unittests",
       "rtc_base/experiments:experiments_unittests",
+      "sdk:sdk_tests",
       "test/scenario/network:network_emulation_unittests",
     ]