floss client: Fix clippy warnings

- warning: this `MutexGuard` is held across an `await` point
- warning: this function has too many arguments (9/7)
- warning: you seem to be trying to use `match` for destructuring a
           single pattern. Consider using `if let`
- warning: use of a disallowed/placeholder name `foo`
- warning: writing `&Vec` instead of `&[_]` involves a new object where
           a slice will do
- warning: `format!` in `println!` args
- warning: name `NONE` contains a capitalized acronym

Bug: 343315863
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: ./build.py --target test
Test: Deploy to Guybrush, played with btclient command
Flag: EXEMPT, Floss-only changes
Change-Id: Ieffd57244636ced2da057555c2de4a0ef8f4a839