Implement standalone fragment for private tab splash screen
The private tab splash screen should use a standalone fragment similar to the current HomeFragment design. Currently, the private tab splash screen is drawn with a data uri, which often causes the toolbar search/address entry to misbehave. Using a fragment instead of a data uri will avoid these problems and allow the design to be made more consistent with the HomeFragment as more flexible (e.g. we can add links and cards, etc).