Close the ImageReader when the session is closed.

Relying on a DisplayListener is clunky and we can't use the
VirtualDispaly.Callback because it's created server-side.

Using the callback proxy by caching the session but need some
additional synchronization because getScreenshot() will throw
if the reader is already closed and it's better to return null

Bug: 439774796
Flag: android.companion.virtualdevice.flags.computer_control_consent
Test: presubmit
Change-Id: I8a9a80234473998db77d7ae8cc3c33c7720769ff