DmfsTask: extract time and recurrence field builders

[sunkup]

Part of #340.

Extract AllDayBuilder, StartTimeBuilder, DueBuilder, DurationBuilder, and RecurrenceFieldsBuilder from the inline code in DmfsTaskBuilder.buildTask(). AllDayBuilder takes over the timezone resolution logic from DmfsTaskBuilder.getTimeZone().

Each builder implements DmfsTaskFieldBuilder and has a corresponding unit test.