-keepclasses is not a proguard option

It looks like proguard is accepting it and make completion to a valid
option even if there's ambiguity beetwen -keepclasseswithmembers and
-keepclasseswithmembernames. Jack is not permitting that kind of
incomplete options, so fixing.

Change-Id: I034e1cb6ecdcd96beec11ac6b30a0baeb1aebba3

proguard.flags

@@ -4,5 +4,5 @@
   public *** newInstance(com.android.emailcommon.provider.Account, android.content.Context);
 }
 
--keepclasses class com.android.email.activity.setup.AccountSetupFinal
--keepclasses class com.android.email.activity.setup.AccountSettingsFragment
+-keep class com.android.email.activity.setup.AccountSetupFinal
+-keep class com.android.email.activity.setup.AccountSettingsFragment