Support in/out of list announcements on Android

[dnfield]

Fixes flutter/flutter#22876

This makes it so that TalkBack will announce in/out of list under the following conditions:

1. The List has a specified number of children for semantics purposes. Without this, we don't know how many columns or rows to tell TalkBack to announce, and without that, TalkBack won't announce in or out of list.
2. The currently focused A11y node is a child of the list or the currently focused A11y node does not have a parent that has implicit scrolling. This is needed because TalkBack only tracks one list at a time, and so if we have multiple or nested lists it would only announce one. The second part of the check is needed because we want to announce out of list if you move focus to a non-list node.

We really should expose semantics properties for row and column counts, to better support multi-dimensional list-like widgets (like GridView, or third party widgets that want to have better scrolling semantics).

I've added a Predicate utility class because even with Lambda support in Java 1.8, we won't be able to use java.util.function.Predicate (only available in API 24+).

[dnfield]

This should be on my fork, not the flutter one. Will reopen.

[dnfield]

Opened from correct fork on #6918