Add ability to clear lifecycleObserver (#798)

* Added ability clear lifecycleObserver

* Added clearing reference on lifecycle

cameraview/src/main/java/com/otaliastudios/cameraview/CameraView.java

@@ -752,13 +752,27 @@ public class CameraView extends FrameLayout implements LifecycleObserver {
      * Sets the lifecycle owner for this view. This means you don't need
      * to call {@link #open()}, {@link #close()} or {@link #destroy()} at all.
      *
+     * If you want that lifecycle stopped controlling the state of the camera,
+     * pass null in this method.
+     *
      * @param owner the owner activity or fragment
      */
-    public void setLifecycleOwner(@NonNull LifecycleOwner owner) {
-        if (mLifecycle != null) mLifecycle.removeObserver(this);
+    public void setLifecycleOwner(@Nullable LifecycleOwner owner) {
+        if (owner == null) {
+            clearLifecycleObserver();
+        } else {
+            clearLifecycleObserver();
             mLifecycle = owner.getLifecycle();
             mLifecycle.addObserver(this);
         }
+    }
+
+    private void clearLifecycleObserver() {
+        if (mLifecycle != null) {
+            mLifecycle.removeObserver(this);
+            mLifecycle = null;
+        }
+    }
 
     /**
      * Starts the camera preview, if not started already.