init: switch from android_ids to getpwnam

Start to move users of android_filesystem_config.h to use the
standard grp and pwd interface functions.

Advantages:
  * one copy of android_ids in libc, removing it from init objects.
  * immediately starts oem_xxx users and groups in rc.
  * future will support _named_ oem ids via backend improvements.

Change-Id: Ib1ae1e0cbdcaaf60deb3759681a6030b615c069c
Bug: 27999086
Signed-off-by: William Roberts <william.c.roberts@intel.com>