-Changed global Rx exception handling to no longer trigger error activity if the exception is undeliverable.

-Added debug settings to force reporting of undeliverable Rx exceptions.
-Changed back MediaSourceManager to use serial disposable for syncing.

app/src/debug/java/org/schabi/newpipe/DebugApp.java

@@ -58,6 +58,12 @@ public class DebugApp extends App {
         Stetho.initialize(initializer);
     }
 
+    @Override
+    protected boolean isDisposedRxExceptionsReported() {
+        return PreferenceManager.getDefaultSharedPreferences(this)
+                .getBoolean(getString(R.string.allow_disposed_exceptions_key), true);
+    }
+
     @Override
     protected RefWatcher installLeakCanary() {
         return LeakCanary.refWatcher(this)