Fix memory access violation in NetworkStack

Fix memory access violation in com_android_networkstack_tethering_BpfUtils.cpp
caused by invalid format string in com_android_networkstack_tethering_BpfUtils_isEthernet.
If rv is not valid, jniThrowException will format a string using
"Unknown hardware address type %s on interface %s". However, rv is an int but here
"%s" is used in fmt string, thus causing access violation of memory.

Use "%d" instead of "%s" for int.

Bug: 188757793
Change-Id: I9d8ec0708efd747e24b3b3ffed5336434d4f64a7