Change most notfications category

Due to androids limited dnd implementation all notifications with
category_alarm will make sound even in dnd mode. Not all DeskClock's
notifications are actually alarms. Change appropriate notifications
to category_event, so that notifications sounds an be muted when
Do not disturb is turned on.

Change-Id: I5a5eb26fac85357e108e3008f7b57f053f81b599