[kcm_mouse] Sync KDE4 config after writing the KF5 settings, not before

Kdelibs4SharedConfig::syncConfigGroup() reads the settings from the
corresponding KF5 config files.
If we don't write them before, it will use the previous settings.

Also fix Kdelibs4SharedConfig::syncConfigGroup() to read the settings
from the specified config file instead of always kdeglobals.

BUG: 348254
BUG: 367074
FIXED-IN: 5.7.4
REVIEW: 128703