Skip the check for the legacy vendor

The default url for key request is null from Android U, so
the new added case will fail on vendor T and vendor S.

To avoid the case fail on previous version of Android U,
we need to add skip check.

Bug: 323712762
Bug: 333344377
Test: atest android.media.drmframework.cts.MediaDrmClearkeyTest#testGetKeyRequestDefaultUrl
Change-Id: I9812222ed4bcc1947858ad7f6ba0ae964b00e775