hammerhead: Add mpdecision to system group

File ownership is set by ueventd. Upstream commit:
bf8fcfa71fd365add34a394cbe38828ce8a558fb

Change-Id: I7e67f6aafce876b07a099120aa1bc760ab254d20
Merge "hammerhead: Add mpdecision to system group" into cm-11.0
Merge branch 'cm-11.0' of https://github.com/CyanogenMod/android_device_lge_hammerhead into cd-4.4