[MediaProjection] Clarify createVirtualDisplay flags javadoc

A client currently has no idea that several of the flags are
overridden based on the type passed into
IMediaProjectionManager#createProjection (generally by SystemUI,
but may be any component with MANAGE_MEDIA_PROJECTION permission).

Bug: 286541207
Test: manual
Change-Id: I43d155c6ac10dfada948606e43e113ab1b302ede