Only set QT_QPA_PLATFORM if it is not already set

If you need to use a QPA library other than libqpalm, then set
QT_QPA_PLATFORM in the environment prior to launching LunaSysMgr and
WebAppMgr.

Open-webOS-DCO-1.0-Signed-off-by: Eric Blade <eric.blade@palm.com>

Change-Id: Ia009eb4216d7232705ac4d397c55edc66ade8ce2