Implemeting target assistants

- Workload's update_result stage has now been broken up into two parts:
  extract_results and update_output. This is to allow the assistant to
  pull output from the target in between the two stages.
- Updated assistant implementations for Linux and Android targets from
  the exisiting code.
- Extended target descriptor code to handle assistants and their
  parameters as well.
- Updated  the target manager to actually make use of the assistants.

wa/framework/job.py

@@ -68,8 +68,11 @@ class Job(object):
 
     def process_output(self, context):
         self.logger.info('Processing output for job {}'.format(self.id))
-        with signal.wrap('WORKLOAD_RESULT_UPDATE', self, context):
-            self.workload.update_result(context)
+        with signal.wrap('WORKLOAD_RESULT_EXTRACTION', self, context):
+            self.workload.extract_results(context)
+            context.extract_results()
+        with signal.wrap('WORKLOAD_OUTPUT_UPDATE', self, context):
+            self.workload.update_output(context)
 
     def teardown(self, context):
         self.logger.info('Tearing down job {}'.format(self.id))