ueventd: convert to use pwd family of functions

Switch ueventd to use the grp.h and pwd.h family of
functions for converting strings to uids. This removes the
need for using android_filesystem_config.h.

Change-Id: I25a6f536bb11683d5cd69a0d7b348d25a272dd3b
Bug: 27999086
Signed-off-by: William Roberts <william.c.roberts@intel.com>