[MountPointObserverCache] Update mounts less frequently

Even on fastest disks, filling up the disk in 10 seconds is unlikely.
Moreover, Plasma's "low disk" warning only polls every minute as well and Dolphin doesn't warn you about the fact that it's full.
Reduces needless wake ups of disks and network.

CCBUG: 398612

Differential Revision: https://phabricator.kde.org/D15507