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.

Change-Id: If6bfceea246929fb2a53ab6334f152e383f16917
Signed-off-by: Courtney Cavin <courtney.cavin@sonymobile.com>
Fix typos and spelling mistakes in README

Change-Id: Iacd3ebacd41b0867f03612787783c1c313fe2af5