Remove context parameter from settings singletons

These are unnecessary since we started using the global application
context throughout the application. The motivation behind this was to
fix a null pointer exception which occurred during a long-lasting
refresh operation in which apparently some parts of the app were killed.
Remove unnecessary if blocks

If these variables are ever null, we have bigger problems.
Format, sort and clean Podcast variables
Remove LogoDownloaded podcast members

This variable is completely useless and can be removed.
Do not filter episodes on episode deletion
Correctly handle podcast logo drawables

Instead of creating a dummy logo if no logo file has been downloaded
yet, and then forgetting all about updating to the real logo later on,
correctly use the real logo as soon as it is available.