Refactor ContactListAdapter to be reusable for other use cases.

This commit introduces an abstract `BaseContactListAdapter` which provides
most of the adapter logic. The original `ContactListAdapter` extends it to
show date and online status of the contacts.

The new `ContactChooserAdapter` which is used for introductions extends
the `ContactListAdapter` and adds logic for graying out contacts from
different identities than the currently used one.

A new `ContactSelectorAdapter` extends the `BaseContactListAdapter` and
allows to select multiple contacts. It offers a method to return a
collection of all selected `ContactId`s.

This commit also sneaks in an animation when the 'Share Forum' button
is clicked.

Closes #292

briar-android/res/layout/list_item_contact.xml

@@ -64,6 +64,7 @@
 				android:layout_height="wrap_content"
 				android:textColor="@android:color/tertiary_text_light"
 				android:textSize="@dimen/text_size_tiny"
+				android:visibility="gone"
 				tools:text="My Identity"/>
 
 		</LinearLayout>