code: public static boolean isAccessTokenValid(String accessToken) {
      return !TextUtils.isEmpty(accessToken) && !(!accessToken.startsWith("pk.")
        && !accessToken.startsWith("sk.") && !accessToken.startsWith("tk."));
    }
docs: Checks that the provided access token is not empty or null, and that it starts with
  the right prefixes. Note that this method does not check Mapbox servers to verify that
  it actually belongs to an account.
  
  @param accessToken A Mapbox access token.
  @return true if the provided access token is valid, false otherwise.
  @since 1.0.0