Both underscores and asterisks work for italics, it has nothing at all to do with the button. Basically you've confused the parser that converts the Markdown to HTML.
You have to escape the underscore with a backslash: *kernel\_config.gz*
Which shows as: kernel_config.gz
I think as long as it's clear you're good to do whatever you want. Like GAThrawn I would leave the keyboard buttons for actual buttons, though I tend to use bold or nothing for them myself. I also tend to use inline code for menu items.
I suppose in an ideal world we would have a standard, or perhaps site-specific ways to indicate various Android UI ...
I normally save the < kbd > tags for actual hardware buttons (or at least buttons that are normally hardware even if they're sometimes soft) like Power, Menu, Home, Back, etc.
For menu options and clicking on items on the screen I'd normally either bold them (or for long options like descriptions on tick boxes "enclose them in quotes".
So for your ...