[A11y] When file is selected, keep focus on checkboxes only in Dropbox/Google Drive

[nqst]

When navigating in the list of Dropbox / Google Drive files, you can focus not only on checkboxes, but on the filenames as well:

While we really need it with the folders, it's unnecessary with the files. Focusing on the checkbox and on the filename is currently doing the same thing — file selection. With files, let's allow focus on checkboxes only. Pressing the Tab key should navigate to the next checkbox for the faster and more pleasant user experience.

Example of an excepted behavior:

Note: It's important to allow focusing on the folder name since clicking/pressing Enter on the folder name performs another action (opening a folder).

[arturi]

//cc @lakesare