Skip health checks when opening device picker

[Christian-Holbrook]

Summary

Adds

• DeviceManager.scan() method for discovery without health checks
• Device picker now uses scan() instead of refresh() on open
• Health checks still occur on refresh button click or device selection
• Remove stale devices via a scan. Set the device online state when it is fresh enough in the cache
• Fix bug with clear button not clearing the cache
• Fix bug with discovered devices not being remembered in the "lastSeenDevices" list on reload

Test plan

• Unit tests for scan() method
• Integration test verifying picker calls scan() not refresh()