proguard: Keep fields of generated proguard files

Inspired from https://github.com/protocolbuffers/protobuf/blob/main/java/lite.md#r8-rule-to-make-production-app-builds-work

Signed-off-by: Aayush Gupta <aayushgupta219@gmail.com>

Author: theimpulson

app/proguard-rules.pro

@@ -39,3 +39,8 @@
 
 ## For some reason NotificationModeConfigFragment wasn't kept (only referenced in a preference xml)
 -keep class org.schabi.newpipe.settings.notifications.** { *; }
+
+# Prevent R8 from stripping or renaming Protobuf internal fields
+-keepclassmembers class * extends com.google.protobuf.GeneratedMessageLite {
+    <fields>;
+}