Replace empty state images with new illustrations

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

briar-android/artwork/il_empty_state_blogs_dark.svg

briar-android/artwork/il_empty_state_blogs_light.svg

briar-android/artwork/il_empty_state_contact_list_dark.svg

briar-android/artwork/il_empty_state_contact_list_light.svg

briar-android/artwork/il_empty_state_forum_list_dark.svg

briar-android/artwork/il_empty_state_forum_list_light.svg

briar-android/artwork/il_empty_state_group_list_dark.svg

briar-android/artwork/il_empty_state_group_list_light.svg

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

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

...android/contactselection/BaseContactSelectorFragment.java

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