Use heartrate-game example for snippets in BTLE overview

heartlistener example will be removed.

Change-Id: Iaa5c5f629cf351b97c18f61b4fab2cbbd8d0744c
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>