Keep Parcelable CREATORs

At least until the base file includes this

Change-Id: I91497f08b8f546080abbf3e0b1e7a5600072619a
diff --git a/proguard.flags b/proguard.flags
index ad87779..7267eab 100644
--- a/proguard.flags
+++ b/proguard.flags
@@ -8,6 +8,11 @@
 
 # keep names that are used by reflection, or from javascript
 
+# TODO(skennedy) Remove this when the base file includes it
+-keep class * implements android.os.Parcelable {
+  public static final ** CREATOR;
+}
+
 -keepclasseswithmembers class com.android.mail.ui.LeaveBehindItem {
   *** setAnimatedHeight(...);
 }