datetime: Fix checking when timesyncd will run on a system

Fixes #2158.

When 'systemctl show' is used see the ConditionResult property, the value is
correct only when the unit has been started. When the unit is not running but
can run, ConditionResult has a value of 'no' leading to an incorrect result.

This in turn leads to _is_time_managed() returning incorrect value once the
service has been stopped. FreedomBox would have noted that daemon can be
enabled/disabled during startup while during attempts to enable it the action
script will think that service can't be enabled/disabled.

Fix this by using a better approach to detect when the service can run. Newer
versions of systemd (likely >=250) have the ability to run 'systemd-analzye
condition --unit=systemd-timesyncd.service' which have been ideal to detect
this. However, --unit option is not available in older versions. Use
systemd-virt-detect (part of systemd package) to detect for containers instead.

Tests:

- Boot the machine and run datetime functional tests

- User interface should not show enable/disable button for the app in container
but show in VM.

- Running first setup (after removing /var/lib/plinth/plinth.sqlite3) should
work on container and VM.

- Run above tests on a container and on a VM

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/datetime/__init__.py

@@ -45,13 +45,16 @@ class DateTimeApp(app_module.App):
         """
         if self._time_managed is None:
             try:
-                output = subprocess.check_output([
-                    'systemctl', 'show', '--property=ConditionResult',
-                    '--value', 'systemd-timesyncd'
-                ])
-                self._time_managed = 'yes' in output.decode()
+                # Replace with the command 'systemd-analyze condition
+                # --unit=systemd-timesyncd.service' when --unit argument
+                # becomes available in a newer systemd.
+                process = subprocess.run(
+                    ['systemd-detect-virt', '--container'], check=False,
+                    stdout=subprocess.PIPE)
+                self._time_managed = (
+                    process.stdout.decode().strip() == 'none')
             except (FileNotFoundError, subprocess.CalledProcessError):
-                # When systemd is not running.
+                # When systemd-timesyncd will not run.
                 self._time_managed = False
 
         return self._time_managed