Issue 6748: Unable to download ZIP (and many other filetypes) files attached to emails

Issue: http://code.google.com/p/cyanogenmod/issues/detail?id=6748

This change allow the user to select which extensions of the UNACCEPTABLE_ATTACHMENT_EXTENSIONS
defined by AOSP should allow/disallow from attachment download.
A new General->Security setting was added with a multi select list preference with
all the extensions. By default all the extensions will be marked as blocked.

Patchset 3: Rename disallowed to blocked. Cleanup.

Change-Id: I19e6ff96d6251c5edc77b89dd8c44db1342677fa