Reduce visibility of UnobservedTaskException

Summary:
In an attempt to reduce footprint of React Native Android public APIs we are reducing visibility of classes and interfaces that are not meant to be used publicly OR are public but have no usages.
As part of our analysis, which involved looking for usages inside the Meta codebase and code search in OSS, we've detected that this class/interface is public but it's not used from other packages.

If you are using this class or interface please comment in this PR and we will restate the public access.

bypass-github-export-checks

changelog: [Android][Changed] Reducing visibility of UnobservedTaskException

Reviewed By: cortinico

Differential Revision: D49803272

fbshipit-source-id: cbd9bd285ba4c2ce5b3e77885da37f125517fac9

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/runtime/internal/bolts/UnobservedTaskException.java

@@ -10,7 +10,7 @@
 import androidx.annotation.Nullable;
 
 /** Used to signify that a Task's error went unobserved. */
-public class UnobservedTaskException extends RuntimeException {
+class UnobservedTaskException extends RuntimeException {
   public UnobservedTaskException(@Nullable Throwable cause) {
     super(cause);
   }