Add a confirmation dialog to empty trash/spam.

Fixes b/8753294. Creates a new EmptyFolderDialogFragment
that displays the dialog, handles orientation changes, and
calls back to AbstractActivityController when delete is selected
to perform the mass deletion. Additionally, we now only show
the empty overflow options if there are conversations in the
folder.

Change-Id: Ibc9ff0b9511a4cdc4c390cd7273706abedb8d9a2
4 files changed