pylint/pep8 fixes

- android/workload: emoved an extra bank line between methods
- trace_cmd: define member attribute inside __init__
- adb_shell: ignore pylint warning about too many branches in this case

wlauto/common/android/workload.py

@@ -169,7 +169,6 @@ class ApkWorkload(Workload):
         self.apk_version = None
         self.logcat_log = None
 
-
     def init_resources(self, context):
         self.apk_file = context.resolver.get(wlauto.common.android.resources.ApkFile(self),
                                              version=getattr(self, 'version', None),