Add option to skip titlescreen, fix hint crash, fix custom themes bug

[jlnordin]

This PR addresses issues #65, #64, and #62 by fixing a few bugs and adding a feature to skip the new title screen and launch directly on the Sudoku list screen.

Skip Titlescreen

Per user feedback, this PR adds a new option "Show sudoku lists on startup" that will launch the app directly to the sudoku lists screen on startup. The title screen will still be accessible via back navigation, but the app will start on the lists activity.

Hint Crash

If you used the "hint" command from the options menu when no cell was selected, the activity would crash. This PR adds a null check to make sure the "couldn't show hint" dialog is shown instead.

Sudoku Preview Colors in Folder View

If you are using a custom theme instead of a built-in theme, the sudoku previews that are shown in the folder views were always black-and-white. This PR changes it to reflect the current custom theme colors.

Scenarios Tested

• Launched into the app with and without the "show sudoku lists on startup" option.
• Verified changelog is displayed on either the title screen or the sudoku lists screen, depending on what option is selected.
• Verified selecting the "hint" command when a cell is not selected shows the hint error dialog (instead of a crash).
• Verified the Sudoku previews in the sudoku folder activity are colored using the colors in the custom theme if the custom theme is selected.
• Verified the Sudoku previews in the sudoku folder activity are colored correctly when using a normal theme.

[jlnordin]

Note: I can split this into 3 different PRs if needed. I kept this as one PR just to save some time when merging in later.

[ogarcia]

Merged! Many thanks for the fixes!