run-as: Keep supplementary groups.

This broke TCP debugging because processes don't inherit the AID_INET
group.

Bug: 67058466
Test: adb shell run-as com.example.native_activity groups prints "inet".
Change-Id: Ieb461dccda8611057bb2d16334e584eb5e57c8b1

run-as/run-as.cpp

@@ -194,6 +194,7 @@ int main(int argc, char* argv[]) {
   ScopedMinijail j(minijail_new());
   minijail_change_uid(j.get(), uid);
   minijail_change_gid(j.get(), gid);
+  minijail_keep_supplementary_gids(j.get());
   minijail_enter(j.get());
 
   if (selinux_android_setcontext(uid, 0, info.seinfo, pkgname) < 0) {