dumpstate: only poll 30s for initial data

Before this patch, dumpstate will poll an fd for 30s per line. If the
fd blocks on poll, then this inserts a 30s sleep, which is bad.

After this patch, dumpstate will poll an fd for 30s only for the
initial read of the fd. All subsequent reads of that fd will return
immediately, and as soon as the fd does not have any available data to
read, the polling will cease.

Test: bug reports work and don't time out on /dev/trusty-log0
Bug: 238220830
Change-Id: Ie34cebac62608a914477978704fef813372a273d