power: Improve battery warning debounce logic
requested to merge intgr/gnome-settings-daemon:power-Improve-battery-warning-debounce-logic into master
Despite the debounce logic introduced in !210 (merged) (60621b90), GSD kept warning me of the battery level every time my Logitech K600 keyboard (via Unifying receiver) disconnects/reconnects.
Added another condition: when battery level drops to
UP_DEVICE_LEVEL_NONE, the previous entry in the hash table should not
be overwritten. This avoids duplicated warnings when transitioning
between NONE->LOW->NONE->LOW.
Edited by Marti Raudsepp