Improve permissions for display proxy methods

Addresses a number of issues with the two methods:

- Checking for CREATE_VIRTUAL_DEVICE permission is redundant as the
  implementations validates that the caller owns the provided displayId
  in a VirtualDevice. Creating a VirtualDevice via VirtualDeviceManager is
  only possible if the caller holds CREATE_VIRTUAL_DEVICE.
- AccessibilityManager#unregisterDisplayProxy did not verify that the
  caller owns the display, letting different VirtualDevice owners
  unregister each others A11yProxies.
- Move the displayBelongsToCaller function to the
  AccessibilitySecurityPolicy as that seems like a better place for the
  function.

Bug: 399840253
Test: atest AccessibilityManagerServiceTest
Flag: EXEMPT refactor
Change-Id: I6ac25bf0bee91a2b8da47a8eefe4797f6c1dd4fc