ueventd: convert platform_names from C list to std::vector

Also simplify this code a bit.

There's only one consumer that removes the /devices/platform prefix,
so have them handle it instead of storing two copies of the string.

Remove an unneeded search for '/' in get_character_device_symlinks()
as a / will always be the next character after a parent path, by
nature of FindPlatformDevice().

Test: boot bullhead
Test: init unit tests

Change-Id: I9d0482d137b1342ae7509ae993ff99198be814f0