libcutils: fix Wsign-compare warnings in multiuser_test

gid_t and uid_t are unsigned ints, so we need to compare against unsigned
constants.

Bug: 26936282
Test: mmm system/core/libcutils/tests
Test: adb shell /data/nativetest/libcutils_test/libcutils_test32 \
	--gtest_filter=MultiuserTest.* (on hikey)
Test: adb shell /data/nativetest64/libcutils_test/libcutils_test64 \
	--gtest_filter=MultiuserTest.* (on hikey)

Change-Id: Ic4c8a137e3f37cb5e95ba925814f201cfd5a06b0
Signed-off-by: Greg Hackmann <ghackmann@google.com>