Remove VisibleForTesting from SDK production code (#7103)

Some methods are currently marked with `@VisibleForTestingOnly` but are
being used directly within the SDK code. This annotation is intended to
restrict usage to tests, and its presence causes errors when integrating
the SDK into internal systems via copybara.

Internal Tracking: b/429425834

Author: tusharkhandelwal8

firebase-config/src/main/java/com/google/firebase/remoteconfig/internal/ConfigFetchHandler.java

@@ -77,7 +77,7 @@ public class ConfigFetchHandler {
    *
    * <p>Defined here since {@link HttpURLConnection} does not provide this code.
    */
-  @VisibleForTesting static final int HTTP_TOO_MANY_REQUESTS = 429;
+  static final int HTTP_TOO_MANY_REQUESTS = 429;
 
   /**
    * First-open time key name in GA user-properties. First-open time is a predefined user-dimension

firebase-config/src/main/java/com/google/firebase/remoteconfig/internal/ConfigSharedPrefsClient.java

@@ -373,7 +373,6 @@ Date getBackoffEndTime() {
   // Realtime exponential backoff logic.
   // -----------------------------------------------------------------
 
-  @VisibleForTesting
   public RealtimeBackoffMetadata getRealtimeBackoffMetadata() {
     synchronized (realtimeBackoffMetadataLock) {
       return new RealtimeBackoffMetadata(
@@ -394,7 +393,6 @@ void setRealtimeBackoffMetadata(int numFailedStreams, Date backoffEndTime) {
     }
   }
 
-  @VisibleForTesting
   public void setRealtimeBackoffEndTime(Date backoffEndTime) {
     synchronized (realtimeBackoffMetadataLock) {
       frcSharedPrefs
@@ -415,7 +413,6 @@ void resetRealtimeBackoff() {
    * <p>The purpose of this class is to avoid race conditions when retrieving backoff metadata
    * values separately.
    */
-  @VisibleForTesting
   public static class RealtimeBackoffMetadata {
     private int numFailedStreams;
     private Date backoffEndTime;