Bug 1105639 - Do not register settings listener multiple times r=kgrandon,arthurcc

SettingsHelper's init() method does add an observer on the setting
value. To avoid leaking, we make sure we are only constructing
SettingsHelper once per setting. We fix this for two observers that were
proved to be leaking: lockscreen.lock-message and powersave.threshold.
Merge pull request #26534 from lissyx/bug1105639

Bug 1105639 - Release observers when SettingsHelper is done r=kgrandon,arthurcc