Refactor doesExist() method.

.../briarproject/briar/android/contact/ConversationItem.java

...rg/briarproject/briar/api/blog/BlogInvitationRequest.java

.../briarproject/briar/api/forum/ForumInvitationRequest.java

...arproject/briar/api/introduction/IntroductionRequest.java

.../org/briarproject/briar/api/messaging/PrivateRequest.java

...r/api/privategroup/invitation/GroupInvitationRequest.java

...org/briarproject/briar/api/sharing/InvitationRequest.java

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

...ivategroup/invitation/GroupInvitationIntegrationTest.java

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