presence: Add fallback redundancy policy

Add a fallback redundancy implementation of the RedundancyPolicy
interface.

The fallback policy associates multiple PresenceSensor instances
to a single fan, and "falls-back" on secondary sensor implementations
when the primary sensor cannot see the fan.

Change-Id: I6468d77d97b8916b3ff33bcd0cd28a102d1aaba1
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
3 files changed