floss service: Fix clippy warnings

- warning: this `MutexGuard` is held across an `await` point
- warning: this function has too many arguments (17/7)
- warning: you seem to be trying to use `match` for destructuring a
           single pattern. Consider using `if let`
- warning: this import is redundant

Bug: 343315863
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: ./build.py --target test
Flag: EXEMPT, Floss-only changes
Change-Id: I55366cf160c3d9f6335756c6d8f1cc1a6b1ae793