Update proguard config to keep base Preference class

b/23419547

Change-Id: Id9963d0033a484a7cae8a08926ad972f90558abf
diff --git a/v7/preference/proguard-rules.pro b/v7/preference/proguard-rules.pro
index 07c2b2b..531302c 100644
--- a/v7/preference/proguard-rules.pro
+++ b/v7/preference/proguard-rules.pro
@@ -13,5 +13,6 @@
 # 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