Use standard content provider instead of TaskProvider in TaskSyncer

[sunkup]

Purpose

Before bitfireAT/ical4android#161 TaskSyncer had to acquire TaskProvider in order to find the local tasks list or create a local task list. Now it can use the normal content provider client from Syncer.

bitfireAT/ical4android#161 did also update the DmfsTaskList to hold a standard content provider instead of a task provider so we need to update its usages as well.

Short description

• Updates TaskSyncer to use the standard content provider from base class Syncer.
• Still checks whether the (Task-) provider is too old.
• Updates LocalTaskList and LocalTask which inherit and use DmfsTaskList to use the standard content provider. TaskProvider held the actual content provider client in it's "client" property. Now we can use the provider directly.

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

[ArnyminerZ]

Looks good to me

[rfc2822]

Nice :)