MainActivity should use ContentResolver for browsing

[ChernyshovYuriy]

Current implementation of adapter to the drawer conflicts with permission handling mechanism at StorageProvider. In fact, usb receiver never receives permission related event because it is handled at StorageManager. For this reason, ContentResolver should be used to get content of granted usb device. Or, to introduce less changes, to listen to root changes from content provider and get is as "device attached / detached". Otherwise, current logic doesn't work as is.