Adds all tombstone files when writing to zipped file.

Currently, a bugreport simply cats any tombstone file modified in the
last half an hour into the bugreport. This is a problem since the
tombstones contain a lot of really valuable information, and sometimes
users don't get a bugreport in this time frame. In addition, some of our
monkey testing has the same problem.

Since now dumpstate can create a zip file, we can include all  directly
on it, although still using the old mechanism when it's creating it (for example, when invoked through 'adb bugreport').

BUG: 25974224
Change-Id: Ie29fd7d91953d91232b0db1c9588043aee13f93e