Prepare private message retrieval through ConversationManager

...arproject/briar/android/contact/ConversationActivity.java

...arproject/briar/api/introduction/IntroductionManager.java

...briarproject/briar/api/messaging/ConversationManager.java

...rg/briarproject/briar/api/messaging/MessagingManager.java

...r/api/privategroup/invitation/GroupInvitationManager.java

...va/org/briarproject/briar/api/sharing/SharingManager.java

...arproject/briar/introduction/IntroductionManagerImpl.java

...briarproject/briar/messaging/ConversationManagerImpl.java

...rg/briarproject/briar/messaging/MessagingManagerImpl.java

...r/privategroup/invitation/GroupInvitationManagerImpl.java

...va/org/briarproject/briar/sharing/SharingManagerImpl.java

...oject/briar/introduction/IntroductionIntegrationTest.java

...ivategroup/invitation/GroupInvitationIntegrationTest.java

...ivategroup/invitation/GroupInvitationManagerImplTest.java

...riarproject/briar/sharing/BlogSharingIntegrationTest.java

...iarproject/briar/sharing/ForumSharingIntegrationTest.java