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