[MediaProjection] Handle null DisplayContent when trying to set the session

The VirtualDisplay may have already been removed by the time WindowManager
attempts to handle an incoming recording session. The returned DisplayContent
is then null.

Bug: 280596594
Test: Manual; no NPE when screenshot from Betterbug
Change-Id: Ie956a6b6fea0efd5bd35afa0f73b415a7f6ca346