ApkWorkload: Support implicit activity path

If the activity field of an instance of ApkWorkload does not the '.'
character, it is assumed that it is in the Java namespace of the
application. This is similar to how activities can be referred to with
relative paths:
    com.domain.app/.activity
instead of
    com.domain.app/com.domain.app.activity

wa/framework/workload.py

@@ -257,6 +257,12 @@ class ApkWorkload(Workload):
                 raise ConfigError('Target does not appear to support Android')
 
         super(ApkWorkload, self).__init__(target, **kwargs)
+
+        if self.activity is not None and '.' not in self.activity:
+            # If we're receiving just the activity name, it's taken relative to
+            # the package namespace:
+            self.activity = '.' + self.activity
+
         self.apk = PackageHandler(self,
                                   package_name=self.package_name,
                                   variant=self.variant,