[RAM] Fix snooze scheduler timezone handling (elastic#157338)

## Summary

Closes elastic#156535 

The Snooze Scheduler was failing to properly save and load snoozes if
the user selected a timezone other than the Kibana default. This is
because the datepicker only converts timestamp values between UTC and
the default Kibana timezone.

This PR fixes the issue by offsetting all dates that come in and out of
the scheduler UI relative to local time.

To test, create a snooze like this on `main` and make sure you select
timezone America/Los_Angeles. (If your local timezone is equivalent to
America/Los_Angeles, select a different timezone)

<img width="426" alt="Screenshot 2023-05-10 at 3 58 57 PM"
src="https://github.com/elastic/kibana/assets/1445834/ab95c47c-30a0-44d5-bd9d-45fdb929193d">

On `main`, editing the snooze will (erroneously) display the wrong
times:

<img width="423" alt="Screenshot 2023-05-10 at 4 03 24 PM"
src="https://github.com/elastic/kibana/assets/1445834/ff28ccdd-f491-43ce-87cd-d606c8d71c64">

Repeating this process on this PR's branch will save a snooze with the
correct `dtstart` and consequently load the correct snooze time.

x-pack/plugins/triggers_actions_ui/public/application/sections/rules_list/components/rule_snooze/scheduler.tsx

@@ -146,9 +146,21 @@ const RuleSnoozeSchedulerPanel: React.FunctionComponent<PanelOpts> = ({
           ...(initialSchedule.rRule.until ? { until: moment(initialSchedule.rRule.until) } : {}),
         } as RecurrenceSchedule);
 
+    // Ensure intitial datetimes are displayed in the initial timezone
+    const startMoment = moment(initialSchedule.rRule.dtstart).tz(initialSchedule.rRule.tzid);
+    const dtstartOffsetToKibanaTz = moment()
+      .tz(defaultTz)
+      .year(startMoment.year())
+      .month(startMoment.month())
+      .date(startMoment.date())
+      .hour(startMoment.hour())
+      .minute(startMoment.minute())
+      .second(startMoment.second())
+      .millisecond(startMoment.millisecond())
+      .toISOString();
     return {
-      startDT: moment(initialSchedule.rRule.dtstart),
-      endDT: moment(initialSchedule.rRule.dtstart).add(initialSchedule.duration, 'ms'),
+      startDT: moment(dtstartOffsetToKibanaTz),
+      endDT: moment(dtstartOffsetToKibanaTz).add(initialSchedule.duration, 'ms'),
       isRecurring,
       recurrenceSchedule,
       selectedTimezone: [{ label: initialSchedule.rRule.tzid }],
@@ -218,6 +230,19 @@ const RuleSnoozeSchedulerPanel: React.FunctionComponent<PanelOpts> = ({
 
   const onClickSaveSchedule = useCallback(() => {
     if (!startDT || !endDT) return;
+
+    const tzid = selectedTimezone[0].label ?? defaultTz;
+    // Convert the dtstart from Kibana timezone to the selected timezone
+    const dtstart = moment()
+      .tz(tzid)
+      .year(startDT.year())
+      .month(startDT.month())
+      .date(startDT.date())
+      .hour(startDT.hour())
+      .minute(startDT.minute())
+      .second(startDT.second())
+      .toISOString();
+
     const recurrence =
       isRecurring && recurrenceSchedule
         ? recurrenceSchedule
@@ -227,8 +252,8 @@ const RuleSnoozeSchedulerPanel: React.FunctionComponent<PanelOpts> = ({
     onSaveSchedule({
       id: initialSchedule?.id ?? uuidv4(),
       rRule: {
-        dtstart: startDT.toISOString(),
-        tzid: selectedTimezone[0].label ?? defaultTz,
+        dtstart,
+        tzid,
         ...recurrence,
       },
       duration: endDT.valueOf() - startDT.valueOf(),