Improve physical keyboard support

### Is your feature request related to a problem? Please describe.
When using physical keyboard (or remote on Android TV) and browsing through files and directories, it is impossible to access Share button and Context menu button (3 dots).

### Describe the solution you'd like
I think it should be sufficient to add support for left/ right arrow keys. When a directory or file is selected (with up/ down arrows), clicking right should select Share button, clicking right again would go to Context menu button.

### Describe alternatives you've considered
Using a mouse or touch screen is an option, of course. But sometimes it's not as easy as one might think (again: Android TV).

### Additional context
If you need more info, please ping me. I'm happy to provide more details.

### Additional info
I can try implementing this myself and provide a pull request - but please point me to relevant parts of application code. I've tried finding them myself, but I'm not Android Java dev so it's hard for me to find a good starting point.