Hidden API to get virtual device policy for a specific display.

This change introduces `getDevicePolicyForDisplayId` in `VirtualDeviceManager` and its service implementation. This allows querying device policies, such as recents and activity launch, for a specific virtual display ID, providing more granular control over window policies within a virtual device.

We allow VDM to override some policies for a specific display, but
there's no way to query display-level policies.

Bug: 437848012
Flag: EXEMPT bugfix
Test: presubmit
Change-Id: Ica8203de37a519df5ab7a8b928aabcc50c7cee6a