Allow DbViewModel work on things other than lists.

...va/org/briarproject/briar/android/blog/BaseViewModel.java

...ava/org/briarproject/briar/android/blog/BlogFragment.java

...va/org/briarproject/briar/android/blog/BlogViewModel.java

...ava/org/briarproject/briar/android/blog/FeedFragment.java

...va/org/briarproject/briar/android/blog/FeedViewModel.java

...briarproject/briar/android/contact/ContactsViewModel.java

.../briarproject/briar/android/forum/ForumListViewModel.java

.../org/briarproject/briar/android/forum/ForumViewModel.java

...iar/android/privategroup/conversation/GroupViewModel.java

...t/briar/android/privategroup/list/GroupListViewModel.java

...org/briarproject/briar/android/viewmodel/DbViewModel.java