Pause Periodic List Refresh when View is not Visible

...org/briarproject/android/contact/ContactListFragment.java

...rg/briarproject/android/contact/ConversationActivity.java

...oid/src/org/briarproject/android/forum/ForumActivity.java

...src/org/briarproject/android/forum/ForumListFragment.java

...droid/src/org/briarproject/android/util/AndroidUtils.java

.../src/org/briarproject/android/util/BriarRecyclerView.java