Added r8 rules to speed up main dispatcher loading...

Added r8 rules to speed up main dispatcher loading (https://github.com/Kotlin/kotlinx.coroutines/issues/1231#issuecomment-499300105)

app/proguard-rules.pro

@@ -34,4 +34,9 @@
 
 -keepclassmembernames class kotlinx.** {
     volatile <fields>;
-}
\ No newline at end of file
+}
+# Ensure the custom, fast service loader implementation is removed.
+-assumevalues class kotlinx.coroutines.internal.MainDispatcherLoader {
+  boolean FAST_SERVICE_LOADER_ENABLED return false;
+}
+-checkdiscard class kotlinx.coroutines.internal.FastServiceLoader
\ No newline at end of file