Simplify alarm alerting screen and fix bug with
alerting on time value change
SetAlerting creates an lv_task to automatically call StopAlerting after one minute. This task will call an invalid function reference and lead to a crash under the following condition:
All exit paths but the time value change (so not considering this fix) call StopAlerting themselves, which also terminates the lv_task.
However, the value change callback only calls DisableAlarm, because its normal use case is for setting up an alarm, where you have to re-confirm enabling the alarm after every change you make.
DisableAlarm still sets isAlerting in the alarmController to false, probably because someone thought a currently alerting but also disabled alarm makes no sense, this was introduced in a0cd439.
That causes the destructor of Alarm to think there is nothing to do regarding the alerting when the alarm screen is dismissed.
Therefore it does not call StopAlerting and the lv_task is left with an invalid function pointer, because Alarm does not exist anymore once the lv_task finally goes to call the callback function
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