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/instrumentation/fps/__init__.py

@@ -70,6 +70,7 @@ class FpsInstrument(Instrument):
                        vsync cycle.
 
     """
+    supported_platforms = ['android']
 
     parameters = [
         Parameter('drop_threshold', kind=numeric, default=5,