Systemd unit fails when getprop returns ""

[Chris Halse Rogers]

When installing bluetooth-touch on the desktop (as it is a dependency of ubuntu-desktop-next):

● bluetooth-touch.service - Bluetooth init for Touch devices
   Loaded: loaded (/lib/systemd/system/bluetooth-touch.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Mon 2015-10-19 09:55:17 AEDT; 22min ago

Oct 19 09:55:17 RedTail systemd[1]: Starting Bluetooth init for Touch devices...
Oct 19 09:55:17 RedTail sh[18984]: /bin/sh: 1: exec: /usr/share/bluetooth-touch/: Permission denied
Oct 19 09:55:17 RedTail systemd[1]: bluetooth-touch.service: Control process exited, code=exited status=126
Oct 19 09:55:17 RedTail systemd[1]: Failed to start Bluetooth init for Touch devices.
Oct 19 09:55:17 RedTail systemd[1]: bluetooth-touch.service: Unit entered failed state.
Oct 19 09:55:17 RedTail systemd[1]: bluetooth-touch.service: Failed with result 'exit-code'.

This is because `getprop ro.product.device` quite unsurprisingly returns "" on my desktop system.

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: bluetooth-touch 0.3.1
ProcVersionSignature: Ubuntu 4.2.0-16.19-generic 4.2.3
Uname: Linux 4.2.0-16-generic x86_64
ApportVersion: 2.19.1-0ubuntu2
Architecture: amd64
CurrentDesktop: Unity
Date: Mon Oct 19 09:21:07 2015
InstallationDate: Installed on 2013-08-06 (803 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
SourcePackage: bluetooth-touch
SystemImageInfo:
 current build number: 0
 device name:
 channel: daily
 last update: 2015-07-02 17:27:08
UpgradeStatus: Upgraded to wily on 2013-08-06 (803 days ago)