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/applaunch/__init__.py

@@ -60,6 +60,7 @@ class ApplaunchWorkload(Workload):
     Measures the time and energy used in launching an application.
 
     """
+    supported_platforms = ['android']
 
     parameters = [
         Parameter('app', default='browser', allowed_values=['calculator', 'browser', 'calendar'],

wlauto/workloads/audio/__init__.py

@@ -33,6 +33,7 @@ class Audio(Workload):
     it plays Canon_in_D_Pieano.mp3 for 30 seconds.
 
     """
+    supported_platforms = ['android']
 
     parameters = [
         Parameter('duration', kind=int, default=30,

wlauto/workloads/dex2oat/__init__.py

@@ -39,6 +39,7 @@ class Dex2oatBenchmark(Workload):
 
         """
 
+    supported_platforms = ['android']
     command_template = 'dex2oat --dex-file={} --oat-file={} --instruction-set={} --dump-timing'
     run_timeout = 5 * 60
 

wlauto/workloads/homescreen/__init__.py

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

wlauto/workloads/power_loadtest/__init__.py

@@ -42,6 +42,7 @@ class PowerLoadtest(Workload):
     See: https://www.chromium.org/chromium-os/testing/power-testing
 
     '''
+    supported_platforms = ['chromeos']
 
     parameters = [
         Parameter('board', default=os.getenv('BOARD'),

wlauto/workloads/video/__init__.py

@@ -45,6 +45,7 @@ class VideoWorkload(Workload):
     .. _Big Buck Bunny: http://www.bigbuckbunny.org/
 
     """
+    supported_platforms = ['android']
 
     parameters = [
         Parameter('play_duration', kind=int, default=20,