service: Implement IBluetooth IsEnabled, Enable, and Disable

Implemented the IsEnabled, Enable, and Disable methods of the IBluetooth
interface. Added a simple REPL logic to client/main.cpp so that these commands
can be tested real-time via command-line.

Bug: 23169366
Change-Id: Id27a82e5cdadc5ea0b6f88d3ab3a6b7882f6212a