Set camera to null when disabling IMS video transmission

From Tyler's investigation:

In the past (Marshmallow days) we used to call setCamera(null) as well
as sending the downgrade request.  Stopping the video transmission is
one thing but I think the modem implementation depends on the camera
being nulled to clean up its resources.

(This CL is a manual cherry pick of cl/157154988 from google3 to
gerrit.)

Bug: 38395481
Change-Id: I7f71c1ffb7194165b8968cd4a033ffea1af443ed
diff --git a/java/com/android/incallui/videotech/ims/ImsVideoTech.java b/java/com/android/incallui/videotech/ims/ImsVideoTech.java
index 363a193..01e4bb8 100644
--- a/java/com/android/incallui/videotech/ims/ImsVideoTech.java
+++ b/java/com/android/incallui/videotech/ims/ImsVideoTech.java
@@ -183,6 +183,8 @@
   public void stopTransmission() {
     LogUtil.enterBlock("ImsVideoTech.stopTransmission");
 
+    setCamera(null);
+
     int unpausedVideoState = getUnpausedVideoState(call.getDetails().getVideoState());
     call.getVideoCall()
         .sendSessionModifyRequest(