tools/btgatt-server: Add Heart Rate service simulation.

This patch adds a Heart Rate service to the database which simulates a
fake Heart Rate service implementation. The service can be enabled by
default by passing the '-r' flag to the executable.
tools/btgatt-server: Add "heart-rate" command.

This patch adds the "heart-rate" command which can be used to hide
or unhide the Heart Rate service. This is useful for testing service
changed events.
tools/btgatt-client: DO NOT SUBMIT: Added "att" command for testing.