Include title and description on bugreport.zip

Prior to this change, the user-provide title and description were only
used in the ACTION_SEND_MULTIPLE intent, which was fine for the cases
where the user share the bug report with an app that used intent
extras (like an email app).

But if the app did not use the extras, or if
the user did not share the bug report right away, the info supplied by
the user would be lost.

With this change, such info will be saved into 2 new zip entries,
title.txt and description.txt

BUG: 26403310
Change-Id: I888364d14d67fb4e2f2c26cb66b21576d7ce13b4