workloads/jankbench: Update to clear logcat using devlib

Leftover code from WA2 meant that logcat was cleared on the device by
the workload directly instead of using devlib, this caused issues if logcat was
still being cleared from other areas of the code.

wa/workloads/jankbench/__init__.py

@@ -208,9 +208,7 @@ class JankbenchRunMonitor(threading.Thread):
         self.daemon = True
         self.run_ended = threading.Event()
         self.stop_event = threading.Event()
-        # Not using clear_logcat() because command collects directly, i.e. will
-        # ignore poller.
-        self.target.execute('logcat -c')
+        self.target.clear_logcat()
         if self.target.adb_name:
             self.command = ['adb', '-s', self.target.adb_name, 'logcat']
         else: