Warn about sensitive bugreport contents.

We show this message when clicking a notification, but we also want
to alert users when they're picking files to attach/share.

Bug: 114711671
Test: manual
Change-Id: I23e249c93d6fa9cf960ef2a61d3fe769eb56ddf5
Merged-In: I23e249c93d6fa9cf960ef2a61d3fe769eb56ddf5
(cherry picked from commit 59137825)