Replace datetime of time_constraints by aware object

Error occurs in alarm evaluation when timezone in time_constraints is specified,
because astimezone() cannot be applied to a native datetime.

Making datetime to non-native (aware object) will solve the problem.

Change-Id: Ic0de4a5b32865bbe330c2936f14adf59168eefe5
Closes-bug: #1342676
Merge "Replace datetime of time_constraints by aware object"