systemd: Rework timing and add After=network-online

The timeout values were so large that the timer could run at any
random time of the day, possibly easily interfering with business
hours, and causing trouble. Reduce them to 30 minutes of random
delay and an accuracy to the default value (1 minute).

Also drop the 18:00 event. People still actively use their device
during that time, and for servers, there might be less attendance
than in the regular 06:00 time slot, so longer time to fix things
if something breaks.

During a boot, the service might be run to catch up with a timer
that would have normally elapsed. Due to no dependencies, it would
have run before the network is online - that's bad. Adding an After
and a Wants fixes that for boots, but still leaves the same issue
for Resume.

LP: #1615482

debian/apt-daily.timer

@@ -1,10 +1,11 @@
 [Unit]
 Description=Daily apt activities
+After=network-online.target
+Wants=network-online.target
 
 [Timer]
-OnCalendar=*-*-* 6,18:00
-RandomizedDelaySec=12h
-AccuracySec=1h
+OnCalendar=*-*-* 6:00
+RandomizedDelaySec=30m
 Persistent=true
 
 [Install]