Update the proguard rules again

Have to explicitly keep the constructors

b/23419547

Change-Id: Iaf06603a475f325161fb56c1d85745181a0131ee
diff --git a/v7/preference/proguard-rules.pro b/v7/preference/proguard-rules.pro
index 531302c..bdcc466 100644
--- a/v7/preference/proguard-rules.pro
+++ b/v7/preference/proguard-rules.pro
@@ -13,6 +13,9 @@
 # limitations under the License.
 
 # Preference objects are inflated via reflection
--keep public class android.support.v7.preference.Preference
--keep public class * extends android.support.v7.preference.Preference
-
+-keep public class android.support.v7.preference.Preference {
+    public <init>(android.content.Context, android.util.AttributeSet);
+}
+-keep public class * extends android.support.v7.preference.Preference {
+    public <init>(android.content.Context, android.util.AttributeSet);
+}