1) Prevent rendering the splash screen as an empty grey shell before
loading image/progress bar.

2) The parent splash screen shell is always visible before image wrapper
was loaded, causing blank shell to be rendered (bug 427393). Hide the
shell until the splash wrapper is created.

Change-Id: I6cd3e608f9e5a5c95bc812341b88f4bd44d1f74f
Signed-off-by: Xi Yan <xixiyan@redhat.com>