Bluetooth device slice improvement

- remove "Pair new device" row
- change the on/off toggle to a plus button for pairing new device
- roll back the title "Bluetooth devices"

Bug: 149667096
Test: robotest
Change-Id: I47e9c47b2ab0adacdbdbde34522d7c0172adda75
Merged-In: I47e9c47b2ab0adacdbdbde34522d7c0172adda75
(cherry picked from commit 5e462a85)