[native] Emit artificial onSelectionChange alongside onChangeText in ClearableTextInput

Summary:
When clearing the input in ClearableTextInput, we construct an artificial onChangeText event. This confuses SelectableTextInput, which expects that onChangeText events should always be accompanied by onSelectionChange events.

As a result of this interaction, a bug occurs because ClearableTextInput defers emiting an onUpdateSyncedSelectionData event until it receives the corresponding onSelectionChange.

This diff addresses the issues by adding an artificial onSelectionChange event alongside the artificial onChangeText event. On iOS onSelectionChange comes first, but on Android it comes last.

Test Plan: Before this change, I had noticed a bug where the @-mention typeahead would not clear after text input cleared. After this change the issue was gone.

Reviewers: tomek, atul

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D6767