chore(R8): Keep the backup models.

Gson will fail with gson.internal.linkedtreemap cannot be cast to class. Mainly because it's removed or obfuscated by R8.

Signed-off-by: KaiserBh <kaiserbh@proton.me>

app/proguard-rules.pro

@@ -80,4 +80,7 @@
 -keep class com.google.api.services.** { *; }
 
 # Google OAuth
--keep class com.google.api.client.** { *; }
+-keep class com.google.api.client.** { *; }
+
+# Keep backup models, otherwise gson fails on release builds.
+-keep class eu.kanade.tachiyomi.data.backup.models.** { *; }