ClatEgress4Key.java - replace U32 ifindex with S32

These are allocated in order by the kernel, if we go
over 2 billion, we've got other problems... besides
U32 to S32 conversion will work just fine anyway.

Surprisingly it appears no further fixups are required
to make this compile, due to other places already being 'int's.

Bug: 245472520
Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I5a48f12e0fc6274debf645a087f5f3857d90be62