ueventd: allow platform devices to have just a /devices/ prefix

When using device tree, platform devices may not have a
/devices/platform/ path prefix, but can be rooted in
/devices/.

Modify the platform device tracking code to store the device
path as well as the name. This way, when we create symlinks, we
can correctly skip the base platform device prefix and get to the
proper device node path.

Change-Id: I939ef8fbcb45c5c803cd9a054e40136a912efc72
Signed-off-by: Dima Zavin <dima@android.com>