Only try to add details to zip once.

When the user enters details (title or description) to the bugreport,
Shell tries to add a title.txt (and/or description.txt) to the zip and
uses 2 instance variables (addedDetailsToZip and addingDetailsToZip) to
control its state.

The problem with the current approach is that if there is a failure
adding the entries (for example, if the entries already exist), these
variables are not updated and hence when the user taps Share, it will
try to add the entries again, which most likely would fail.

BUG: 28291423
Change-Id: I56a71256be4f8de15f8126b815334277319e8e8a