Only register VirtualDeviceListener when a11y proxies are registered

Immediately registering VirtualDeviceListener during the
AccessibilityManagerService start can lead to issues as this depends on
the timing of VirtualDeviceManagerService starting.

Instead, only register VirtualDeviceListener when a a11y proxy is
registered.

Bug: 302519290
Test: atest ProxyManagerTest AccessibilityDisplayProxyTest
Change-Id: I80b5213107107de358adcf5d8193fa438ffe55c0