Replaced InvitationManager with a generic ReferenceManager for Android.

briar-android/src/net/sf/briar/android/AndroidModule.java

...ndroid/src/net/sf/briar/android/ReferenceManagerImpl.java

...c/net/sf/briar/android/invitation/AddContactActivity.java

briar-api/src/net/sf/briar/api/android/ReferenceManager.java

...pi/src/net/sf/briar/api/invitation/InvitationManager.java

...r-api/src/net/sf/briar/api/invitation/InvitationTask.java

...rc/net/sf/briar/api/invitation/InvitationTaskFactory.java

briar-core/src/net/sf/briar/invitation/ConnectorGroup.java

briar-core/src/net/sf/briar/invitation/InvitationModule.java

...rc/net/sf/briar/invitation/InvitationTaskFactoryImpl.java