Update sample ProGuard config to mark fields instead of all members. (#1531)

GSON only needs to reflect based on fields:
4d942db168/gson/src/main/java/com/google/gson/internal/bind/ReflectiveTypeAdapterFactory.java (L152)

There's no reason to disallow optimizing methods.

examples/android-proguard-example/proguard.cfg

@@ -11,7 +11,7 @@
 #-keep class com.google.gson.stream.** { *; }
 
 # Application classes that will be serialized/deserialized over Gson
--keep class com.google.gson.examples.android.model.** { *; }
+-keep class com.google.gson.examples.android.model.** { <fields>; }
 
 # Prevent proguard from stripping interface information from TypeAdapterFactory,
 # JsonSerializer, JsonDeserializer instances (so they can be used in @JsonAdapter)