Properly stop the navigation session

Description

Terminates the navigation service when the user leaves turn-by-turn. This destroys the navigation notification immediately and prevents it from returning. This was tested with and without the developer option "Don't keep activities". With the setting active, the navigation session is persisted even when the activity is destroyed by the system. Moreover, the activity is recreated without any issues when the user taps the notification. It also works as expected without the developer setting.

Fixes #31 (closed)