Adds DESTROY_CONTENT_ON_REMOVAL to VirtualDisplay flags

Currently when an ActivityView is removed/released, any remaining
tasks on the display are moved to the main display. This change
causes those tasks to be destroyed instead.

Bug: 111236845
Test: manual, built and tested with ActivityView SystemUI plugin
Change-Id: Ifaa9df6fab05ce82e1bad4e2f5a19c4c7476aec6