Implement new empty state icons

briar-android/artwork/empty_state_blog.svg

briar-android/artwork/empty_state_contact_list.svg

briar-android/artwork/empty_state_forum_list.svg

briar-android/artwork/empty_state_group_list.svg

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

...iarproject/briar/android/contact/ContactListFragment.java

...g/briarproject/briar/android/forum/ForumListFragment.java

...ct/briar/android/privategroup/list/GroupListFragment.java

...rg/briarproject/briar/android/view/BriarRecyclerView.java

briar-android/src/main/res/drawable/ic_empty_state_blog.xml

...oid/src/main/res/drawable/ic_empty_state_contact_list.xml

...droid/src/main/res/drawable/ic_empty_state_forum_list.xml

...droid/src/main/res/drawable/ic_empty_state_group_list.xml

briar-android/src/main/res/layout/briar_recycler_view.xml

briar-android/src/main/res/layout/fragment_forum_list.xml

briar-android/src/main/res/values/attrs.xml

briar-android/src/main/res/values/strings.xml