Enable the Grue sensor plugin no matter what other sensor plug-in is installed

If multiple plugins with the same key are installed only the first
one is used and any subsequent plugin with the same key is ignored.
This prevents the Grue example from working as it requires a Light
sensor and the Grue sensor. All other plug-ins use the "unused" key.

Change-Id: I3cb28d3ffec407fb27b4550521acf28bd9c7b073
Reviewed-by: Lorn Potter <lorn.potter@jollamobile.com>