Clean up SslErrorHandlerImpl

- Use assert rather than junit.framework.Assert
- Add some comments
- There's no need for checkSslPrefTable() to call handleSslErrorResponse() as
  we'll never update the table. Instead call
  LoadListener.handleSslErrorResponse() directly.

No functional change.

Bug: 5409251
Change-Id: I0c6cdae43fa966f86f4a6c43b74c2f2a01f60319