Everything you need to know:
In summary, you generally don't need to worry about resolution at all, full screen scaling will take care of that for you. There are too many resolutions to target. Note that some phones have higher resolution than pc monitors!
What you do need to take into account and design for is different aspect ratios, by dynamically positioning your static hud elements, and also deciding what parts of your layout/background are acceptable to either get cut off or show extra of.