Bug 1034735 -  [System][OTA] When the device is plugged-in, it should not block OTA even the energy is lower than 50%

r=etienne
Small nit: please add a comment, for example
```
# LOCALIZATION NOTE (systemUpdateLowBatteryThreshold): {{ threshold }} will be
# replaced at runtime by the level of the battery, resulting for example in
# "You need at least 40% battery level to install the update."
```