Refactor LegacyAndroidEventBuilder2 #62

One core task of the synctools rewrite project is to refactor the LegacyAndroidEventBuilder2 to meet the following goals:

• Only operation: mapping input data class (AssociatedEvents) → output data class (EventAndExceptions). (No database operations in the mapper.)
• At the end, every (target) field should have its own mapping class. So for instance a TitleBuilder that is only responsible for building the Events.TITLE field. (See New event builders #50 and Add new event Entity builder logic that is used beside legacy builder #51.)
• For sub-rows like attendees, there should probably be one builder for the Attendee and then again one builder class for every field of the Attendee row.
• Because the only operation is mapping, we can have detailed tests with Roboelectric. And because every field has its own class, we can provide all test cases in the corresponding test class without mixing it with other fields.
• Every field should support everything that's possible with the Android calendar provider. Most fields already have a good functional coverage, but there's still room for improvements (for instance with attendees).

Inputs are:

• the data to map: should be passed to the mapping function (like function build(AssociatedEvents): Entity) and
• context data: should be passed to the constructor of the mapper or the respective field mappers. For instance, mapping requires the eTag or syncId to fill the appropriate fields. These are not coming from the iCalendar itself, but from the sync process → context data → passed in constructor.

Output is always the complete EventAndExceptions data class.

Optimizing field builders (for instance improving RRULE and timezone processing) should be done in separate issues.