Skip tests when the drm scheme is not installed

Not all drm hal implmentations will be present on every device,
but the vendor modules will be. This change allows tests to be skipped
when a drm hal implementation isn't present.

Test: drm vts tests pass with widevine on a gms
device (tests executed) and tests pass on an aosp
device (tests skipped).

Change-Id: I0b87f501a45103960193279eb5b4b2cd1c113acd