Add common proguard rules (google-gemini#141)

When we migrated to common, our types got moved and the proguard rule no
longer applied. I think this should be adequate to handle all of our
serialized types, but if there are more outside of common that'll also
need to be kept.

common/consumer-rules.pro

@@ -19,3 +19,5 @@
 # If you keep the line number information, uncomment this to
 # hide the original source file name.
 #-renamesourcefileattribute SourceFile
+
+-keep class com.google.ai.client.generativeai.common.** { *; }