Refactored finishOnUiThread() into superclass.

...r-android/src/org/briarproject/android/BriarActivity.java

...rg/briarproject/android/contact/ConversationActivity.java

...arproject/android/contact/ReadPrivateMessageActivity.java

...rproject/android/contact/WritePrivateMessageActivity.java

...g/briarproject/android/groups/ConfigureGroupActivity.java

.../org/briarproject/android/groups/CreateGroupActivity.java

...id/src/org/briarproject/android/groups/GroupActivity.java

...oid/src/org/briarproject/android/groups/GroupAdapter.java

...rg/briarproject/android/groups/ReadGroupPostActivity.java