Invoke SetNTP and UpdateUseNtpField in time manager

1. When time mode is changed, invoke systemd timedate1's SetNTP
method to update NTP settings;
2. When settings use_dhcp_ntp property is changed, invoke
NetworkManager's UpdateUseNtpField method to update its setting;
3. Move the common code of getProperty() into utils.hpp

Change-Id: I981e0e9de9c0430282b3364c38c282312bb2be89
Signed-off-by: Lei YU <mine260309@gmail.com>
4 files changed