Add a contentDescription to splitscreen's first staged task

Currently, when we stage the first app in a split, the floatingTaskView has no contentDescription and is announced by screen readers as "unlabeled".

This CL allows FloatingTaskView to copy a contentDescription from the underlying task or ItemInfo when it is created.

Also changes SplitConfigurationOptions#itemInfo to a private field with a public getter for mockito compatibility.

Fixes: 319783133
Test: Confirmed with TalkBack on Overview, contextual and Intent splits
Flag: EXEMPT bugfix
Change-Id: Ia5a95e403bfe8623e17d2ceac9073ef15c7bb476