Fix reselection issue after the text is cleared (#183545)

Fixes https://github.com/flutter/flutter/issues/182927

This PR is to fix a Autocomplete re-selection issue when an option is
cleared up and re-selected again. When `_select()` is called,
`_selecting` is set to `true` and the `_textEditingController` is
updated which caused `_onChangedField()` be called but returned early.
In this case, we should update `_lastFieldText` to the `newSelection`.

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