AphrontFormPolicyControl: Reuse existing control ID
Summary:
Do not unconditionally create a new ID which breaks the accessibility label.
Parent `AphrontFormControl::render()` set an ID via `$this->setID(celerity_generate_unique_node_id())` and then calls `$this->renderInput()` on its subclass `AphrontFormPolicyControl` which unconditionally runs `$control_id = celerity_generate_unique_node_id()` to assign another ID and then does `'id' => $control_id`.
This breaks accessibility (AphrontFormControl's `label for="UQ0_xx"` is now off by one from AphrontFormPolicyControl's `a id="UQ0_xx"`).
Similar existing code can be found in `AphrontFormTokenizerControl::renderInput()` at https://we.phorge.it/source/phorge/browse/master/src/view/form/control/AphrontFormTokenizerControl.php;bccd4f5981a7e2c347e8e26cf24d8394c882724f$57-61
Closes T16019
Test Plan: Create a Maniphest Task form which exposes policy controls. Check the resulting HTML if `<label for="">` and `<a id="">` match.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16019
Differential Revision: https://we.phorge.it/D25910