report & show agent health; color-code flaky successes

[yjbanov]

• Add more health checks:
  • can we talk to all connected devices via adb?
  • do we have at least one healthy Android device?
  • can we connect to Firebase?
  • can we connect to Cocoon backend? (relevant for pre-flight check only; if fails while running CI we won't be able to upload the health check to Cocoon)
• Upload health status to Cocoon before each task
• Show agent health status on the dashboard (and track health check age)
• Color code successful tasks that succeeded after >1 attempts (i.e. flaky)
• Fix Firebase health check (we never read the value and always succeeded)

Fixes https://github.com/flutter/cocoon/issues/15

/cc @cbracken

[cbracken]

buf.writeln(...)?

[yjbanov]

Done.

[cbracken]

lgtm other than a couple minor nits!