base

Add rudimentary keyboard operation support.

Currently only select-all/copy/paste of files is supported.
Note, I wasn't able to push all code into DirectoryFragment,
as Jeff Sharkey suggested, as it doesn't have direct access
to KeyEvents. Bridged this from StandaloneActivity.

Change-Id: I082c457c9c4372e689d3acbca9ae11cd5c554a63