Implement undo deletion.

Add a snackbar that pops up upon deleting files, giving the user a chance to
undo the operation.  If the user takes no action, the files are deleted.

This feature basically hides the files temporarily and pops up the said
snackbar, and if the user doesn' hit "undo", the files will be actually
deleted when the snackbar times out.

BUG=23518578

Change-Id: I181395aff967e62ca9287ed6fea2c7c8b8c09a71