Extend device with sleep functionality

This changeset adds the ability to sleep on the device via a
device.sleep() method. This invokes sleep on the target device. This
is useful for situations where the passage of time on the target
device does not match that of the host, e.g., gem5.

This changeset also updates a number of workloads to use this new
sleep method.

wlauto/workloads/idle/__init__.py

@@ -15,8 +15,6 @@
 
 # pylint: disable=E1101
 
-import time
-
 from wlauto import Workload, Parameter
 from wlauto.exceptions import WorkloadError, ConfigError
 
@@ -56,13 +54,13 @@ class IdleWorkload(Workload):
             self.device.execute('stop && sleep {} && start'.format(self.duration),
                                 timeout=timeout, as_root=True)
         else:
-            time.sleep(self.duration)
+            self.device.sleep(self.duration)
 
     def teardown(self, context):
         if self.stop_android:
             self.logger.debug('Waiting for Android restart to complete...')
             # Wait for the boot animation to start and then to finish.
             while self.device.execute('getprop init.svc.bootanim').strip() == 'stopped':
-                time.sleep(0.2)
+                self.device.sleep(0.2)
             while self.device.execute('getprop init.svc.bootanim').strip() == 'running':
-                time.sleep(1)
+                self.device.sleep(1)