Hide FirebaseAuth javadoc.

The issue is that even though the c.g.firebase.internal package is
hidden with @hide javadoc comment, doclava does not "see" it when
rendering the inherited methods section of the implementor. So we need
an explicit @hide on the interface and all of its methods.

Bug: 80714895
Change-Id: I54e0f5bfa96f535c2df404b9c7ffab591732abb6

Author: vkryachko

firebase-common/src/main/java/com/google/firebase/internal/InternalTokenProvider.java

@@ -8,6 +8,8 @@
 /**
  * Provides a way for {@link com.google.firebase.FirebaseApp} to get an access token if there exists
  * a logged in user.
+ *
+ * @hide
  */
 @KeepForSdk
 public interface InternalTokenProvider {
@@ -18,6 +20,7 @@ public interface InternalTokenProvider {
    * @param forceRefresh force refreshes the token. Should only be set to <code>true</code> if the
    *     token is invalidated out of band.
    * @return a {@link Task}
+   * @hide
    */
   @KeepForSdk
   Task<GetTokenResult> getAccessToken(boolean forceRefresh);
@@ -27,6 +30,7 @@ public interface InternalTokenProvider {
    *
    * @return the String representation of the UID. Returns null if FirebaseAuth is not linked, or if
    *     there is no currently signed-in user.
+   * @hide
    */
   @Nullable
   @KeepForSdk