Run legacy backup tasks on thread pool.

- https://github.com/UweTrottmann/SeriesGuideInternal/issues/90
- Also do some code cleanup.
Run data liberation tasks on thread pool.

- https://github.com/UweTrottmann/SeriesGuideInternal/issues/90
- Should avoid blocking by long running tasks on serial executor.
Push version to 15.0.6, 15008.
Display backup path and database version in legacy backup tool.

- https://github.com/UweTrottmann/SeriesGuideInternal/issues/88
- Drop unused RTL layout.
(cherry picked from commit 400f979)
Only schedule next episode update task if last one is finished.

- Only for all shows update task. Should prevent blocking serial executor with boat loads of this task.
Execute legacy backup tasks on serial executor.

- More likely to avoid conflicts with tasks accessing the database, as the access here is file-based.
Execute backup, next episode tasks on pool. Import on serial executor.

- More clearly label serial executor tasks by using Utils.executeInOrder.
Rename AndroidUtils.executeAsyncTask to .executeOnPool.
Push version to 15009.
Change log for 15.0.6.
Merge branch 'fix-15.0.6'