app_server: Fix potential infinite loop

Return `B_ENTRY_NOT_FOUND` instead of -1 when `/dev/graphics`
is not found.

Otherwise, `app_server` would run into an infinite loop
while waiting `fCardFD` to equal `B_ENTRY_NOT_FOUND` in some
specific environments such as a `chroot` where `/dev` is missing.

Change-Id: Ice23a82f58811f1258c58826c2488ae5c5c29cee
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6376
Reviewed-by: X512 <danger_mail@list.ru>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>

app_server: Move DWindowHWInterface and ViewHWInterface to a subdirectory.

AccelerantHWInterface and the Remote engines already did this.
These are not used at all outside of test_app_server, so we do not
need to even compile them in to the regular one.