Moved PrivateGroupFactory to clients package.

...src/org/briarproject/api/clients/PrivateGroupFactory.java

briar-core/src/org/briarproject/clients/ClientsModule.java

...src/org/briarproject/clients/PrivateGroupFactoryImpl.java

...e/src/org/briarproject/forum/ForumSharingManagerImpl.java

.../src/org/briarproject/messaging/MessagingManagerImpl.java

...briarproject/properties/TransportPropertyManagerImpl.java

briar-core/src/org/briarproject/sync/SyncModule.java

briar-tests/src/org/briarproject/sync/ConstantsTest.java

...rg/briarproject/sync/SimplexMessagingIntegrationTest.java