Add dropbox entries as files to dumpstate ZIP.

Since each dropbox entry is already stored as a file on disk, include
them as-is into the dumpstate ZIP file.

The dumpsys output has already included truncated versions of all
dropbox entries for many years, and adding them as separate files
inside the dumpstate ZIP will speed up debugging and issue triage.

Bug: 267673062
Test: manual
Change-Id: I5a45e39012cebc5fea1e2d0be3146519de7cafd8