ueventd: fix subsystem list logic issues

1) Check subsystems list before doing usb subsystem logic.  This allows
   developers to handle usb* subsystems in ueventd.rc files.
2) Fix a bug where each subsystem_ instance is not reinitialized, but
   rather only the name_ member was set.

Test: boot bullhead
Test: check that multiple uevent_devname subsystems work when
      specified in ueventd.rc
Change-Id: Ifcac04763afcaf72a3b14ef5f3a6cb89981b51a1
diff --git a/init/devices.h b/init/devices.h
index dd44337..1f8f1e8 100644
--- a/init/devices.h
+++ b/init/devices.h
@@ -72,6 +72,7 @@
     friend class SubsystemParser;
 
     Subsystem() {}
+    Subsystem(std::string name) : name_(std::move(name)) {}
 
     // Returns the full path for a uevent of a device that is a member of this subsystem,
     // according to the rules parsed from ueventd.rc