Update proguard rules.

app/proguard-rules.pro

@@ -14,8 +14,16 @@
 
 # Uncomment this to preserve the line number information for
 # debugging stack traces.
-#-keepattributes SourceFile,LineNumberTable
+-keepattributes SourceFile,LineNumberTable
 
 # If you keep the line number information, uncomment this to
 # hide the original source file name.
-#-renamesourcefileattribute SourceFile
+-renamesourcefileattribute SourceFile
+
+## --------------- GOOGLE API ---------------
+
+-keepclassmembers class * {
+  @com.google.api.client.util.Key <fields>;
+}
+-keep class com.google.api.services.sheets.** { *; }
+-keep class com.fasterxml.** { *; }