adding missing supported_platforms attributes.

Bbench only works on Android. It should advertise that fact by setting
supported_platforms to ['android'].
Telemetry is a Chrome browser workload that is only supported on
ChromeOS and Android.

wlauto/workloads/telemetry/__init__.py

@@ -88,6 +88,8 @@ class Telemetry(Workload):
 
     """
 
+    supported_platforms = ['android', 'chromeos']
+
     parameters = [
         Parameter('run_benchmark_path', default=None,
                   description="""