ueventd: remove PlatformDeviceList

In order to create symlinks for USB and block devices, the path for
their parent platform device must be known.

Previously, ueventd would save each platform device that it encounters
to a list and query this list when creating the symlinks.  That,
however, is racy because the uevent socket does not differentiate
uevents from device_init() and uevents sent by the kernel when
probing a device first the first time.  The below scenario is the
faulty case:

1) Kernel probes parent platform device for a block device
2) ueventd calls device_init() and starts processing uevents
3) Kernel probes block device and sends its uevents
4) ueventd picks up the block device uevent during its uevent processing,
   without yet regenerating the platform device uevent, causing improper
   symlinks to be created.

This change stops storing the platform devices in a list, and instead
traverses up the directory structure for each USB or block device
until it reaches a platform device, defined as one whose subsystem is
the platform bus.  This fixes the race and simplifies the ueventd
code.

Bug: 62436493
Bug: 62681642
Test: Boot bullhead
Test: Boot sailfish
Test: Boot hikey + hotplug/unplug sdcard
Merged-In: I21636355d8e434f30e0cba568598a6cf139e67f9
Change-Id: I21636355d8e434f30e0cba568598a6cf139e67f9