Do not start encoding thread in camera preview mode

Signed-off-by: Leo Ma <begeekmyfriend@gmail.com>

library/src/main/java/net/ossrs/yasea/SrsPublisher.java

@@ -257,7 +257,9 @@ public class SrsPublisher {
         } else {
             mEncoder.setCameraFrontFace();
         }
+        if (mEncoder != NULL && mEncoder.isEnabled()) {
                 mCameraView.enableEncoding();
+        }
         mCameraView.startCamera();
     }