list command: can now filter results by supported platform

Added -p option to the list command. This alows filtering results by
supported platforms, e.g.

	wa list workloads -p linux

Also adding missing supported_platforms attribute to various extensions.
If an extension does not have this attribute, the assumption is that it
is supported by all available platforms.

wlauto/workloads/homescreen/__init__.py

@@ -28,6 +28,7 @@ class HomeScreen(Workload):
     specified duration.
 
     """
+    supported_platforms = ['android']
 
     parameters = [
         Parameter('duration', kind=int, default=20,