Reuse running adb instead of killing existing one. Fixes #464

This patch reuses a running `adb` process if one exists.

Tested on Arch Linux, Ubuntu, Windows, and OSX. To test, I ran the simulator add-on with `adb logcat` running in another terminal. And again without `adb logcat`. When the module unloads and `adb` was running on startup, `adb` will be killed. When the module unloads and `adb` was not running on startup, `adb` won't be killed.