sensors: don't mess with -1 in FD_SET/FD_ISSET

DASH seems to have a tendency to pass -1 to FD_SET/FD_ISSET for disabled
sensors.  Bionic will no longer silently drop these, and now aborts upon
encountering them.

This commit has been tested for 2 weeks on Open Devices AOSP, and resolves the issue of crashing Android every 10 minutes or so.