support all Android special accounts

Currently only six of the special Android accounts are supported:  root, adb, media, vpn, gps, and shell.  This series adds support for all of the (current) special Android accounts (as listed in [the Android source code](https://android.googlesource.com/platform/system/core.git/+/master/include/private/android_filesystem_config.h)), and does some code cleanup.