Fix background replacement crash in camcorder.

Add proguard flag to prevent methods from removing.

bug:7106963
Change-Id: I0e7372c3091dd02d4b475924d06b033717ca2028
diff --git a/proguard.flags b/proguard.flags
index 098a540..7cd6c19 100644
--- a/proguard.flags
+++ b/proguard.flags
@@ -25,6 +25,12 @@
   public void onProtectiveCurtainClick(...);
 }
 
+-keep class com.android.camera.VideoModule {
+  public boolean isRecording();
+  public void onCancelBgTraining(...);
+  public void onProtectiveCurtainClick(...);
+}
+
 -keep class * extends android.app.Activity {
   @com.android.camera.OnClickAttr <methods>;
 }