use recently-paired datetime as pill last-seen

[kingshyg]

Possible scenarios to test:

• [x] new pill, no heartbeat - show paired-time as last-seen
• [x] heartbeat time more recent than paired time - show heartbeat time
• [x] new pill, no heartbeat for >80 mins - show null
• [x] old pill, no new heartbeat yet - show paired time
• [x] old pill paired more than 80 mins ago, no new heartbeat - show super-old heartbeat time

https://trello.com/c/azTC48zz/133-update-pill-last-seen-upon-pairing-with-sense

cc @pims @prishil