Add a Proguard rule to keep Protobuf classes as-is.

AndroidX Security delegates crypto operation to Tink which
depends on Protobuf Javalite. Recently Protobuf Javalite
introduced a change that relies on reflection, which doesn't
work with Proguard.

This change adds a rule that keeps the (shaded) Protobuf
classes in Tink as-is.

See also:
 - https://buganizer.corp.google.com/issues/154315507
 - tink-crypto/tink#361
 - protocolbuffers/protobuf#6463
 - https://b.corp.google.com/issues/144631039

app/proguard-rules.pro

@@ -19,3 +19,17 @@
 # If you keep the line number information, uncomment this to
 # hide the original source file name.
 #-renamesourcefileattribute SourceFile
+
+# AndroidX Security delegates crypto operation to Tink which
+# depends on Protobuf Javalite. Recently Protobuf Javalite
+# introduced a change that relies on reflection, which doesn't
+# work with Proguard.
+# This rule keeps the (shaded) Protobuf classes in Tink as-is.
+# See also:
+# - https://github.com/google/tink/issues/361
+# - https://github.com/protocolbuffers/protobuf/issues/6463
+# - https://b.corp.google.com/issues/144631039
+-keep class * extends com.google.crypto.tink.shaded.protobuf.GeneratedMessageLite {
+  *;
+}
+