update proguard rules to fix crash in minify builds

* square/retrofit#3751

app/proguard-rules.pro

@@ -18,4 +18,17 @@
 
 # If you keep the line number information, uncomment this to
 # hide the original source file name.
-#-renamesourcefileattribute SourceFile
+#-renamesourcefileattribute SourceFile
+
+# https://github.com/square/retrofit/issues/3751
+# With R8 full mode generic signatures are stripped for classes that are not
+# kept. Suspend functions are wrapped in continuations where the type argument
+# is used.
+-keep,allowobfuscation,allowshrinking class kotlin.coroutines.Continuation
+
+# R8 full mode strips generic signatures from return types if not kept.
+-if interface * { @retrofit2.http.* public *** *(...); }
+-keep,allowoptimization,allowshrinking,allowobfuscation class <3>
+
+# With R8 full mode generic signatures are stripped for classes that are not kept.
+-keep,allowobfuscation,allowshrinking class retrofit2.Response