Move ContentProvider operations to background thread

[UweTrottmann]

This is a long-term goal. Do this when changing affected code.

Some ContentProvider operations do not run on a background thread. Move them to one (plus switching to Room APIs) and then remove .allowMainThreadQueries() from SgRoomDatabase.

Related to #581.

Make liberal use of coroutines (but with Global scope to ensure db ops run even if activity/fragment scope is cancelled).

[UweTrottmann]

Started with part of ShowTools. 7c692c8c 36fa4b82