Use uid_t for user IDs and gid_t for group IDs

This patch makes the intent of the code more clear without changing any
functionality since uid_t and gid_t are both aliases for uint32_t on
Android systems. See also
https://cs.android.com/android/platform/superproject/+/master:bionic/libc/include/sys/types.h



Bug: 213617178
Test: Compile-tested only.
Change-Id: Ib5012456a7357e79368f00e71e9a280fd6f96063
Signed-off-by: Bart Van Assche <bvanassche@google.com>