Contact can't be removed if introduction client's local group doesn't exist
While testing the introduction UI, I tried to remove an old contact. The "Contact deleted" toast was shown but the contact wasn't removed because the introduction manager's RemoveContactHook threw an exception:
```
04-01 12:35:57.843 1951-8113/org.briarproject W/ConversationActivity: org.briarproject.api.db.NoSuchGroupException
org.briarproject.api.db.NoSuchGroupException
at org.briarproject.db.DatabaseComponentImpl.getMessageMetadata(DatabaseComponentImpl.java:412)
at org.briarproject.clients.ClientHelperImpl.getMessageMetadataAsDictionary(ClientHelperImpl.java:193)
at org.briarproject.introduction.IntroductionManagerImpl.removingContact(IntroductionManagerImpl.java:157)
at org.briarproject.contact.ContactManagerImpl.removeContact(ContactManagerImpl.java:152)
at org.briarproject.contact.ContactManagerImpl.removeContact(ContactManagerImpl.java:109)
at org.briarproject.android.contact.ConversationActivity$14.run(ConversationActivity.java:481)
at org.briarproject.android.BriarActivity$4.run(BriarActivity.java:154)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
at java.lang.Thread.run(Thread.java:841)
```
issue