floss: Refactor btmanagerd to get rid of select!

Refactor the select! in btmanagerd and replace it with a single mpsc
listener loop. This is more in line with what we do on btadapterd and
simplifies other implementations.

Bug: 193261630
Test: Unit tests and verify via btclient on ChromeOS
Tag: #floss
Change-Id: Ib993c4d8ae01cfe34f485a214d8c8cc8d05d1417