floss: Run freshness check in the background forever

Currently Floss only starts freshness check after discovering, but
socket connection and scanner could also add devices into found_devices
without triggering freshness check.

Since the check is low-cost, we decide to run it in the background
forever to make sure all temporary devices are cleared.

Bug: 306091662
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: Verified device is cleared after incoming connection ended
Test: Verified device is cleared after adv lost for a while
Change-Id: Ibc1a8008522a6fb97067422370d4163fc842071a