build: keep proto-generated files

Fixes, an issue, where the proto-generated files where removed by
proguard, leading to an error.

Ref: TeamNewPipe/NewPipeExtractor#1104

Author: FineFindus

app/proguard-rules.pro

@@ -5,6 +5,7 @@
 
 ## Rules for NewPipeExtractor
 -keep class org.schabi.newpipe.extractor.timeago.patterns.** { *; }
+-keep class org.schabi.newpipe.extractor.services.youtube.protos.** { *; }
 ## Rules for Rhino and Rhino Engine
 -keep class org.mozilla.javascript.* { *; }
 -keep class org.mozilla.javascript.** { *; }