Use public API Network#getNetId() to get netid

Network class will be a part of mainline module, external callers
cannot call its hidden constant. Use public API - getNetId() to
get netid instead.

Bug: 182963397
Test: m
Change-Id: I6d5dd0a8c3879df7bdc1d58e36022045c303bb40

services/core/java/com/android/server/IpSecService.java

@@ -1651,7 +1651,7 @@ public class IpSecService extends IIpSecService.Stub {
                         c.getMode(),
                         c.getSourceAddress(),
                         c.getDestinationAddress(),
-                        (c.getNetwork() != null) ? c.getNetwork().netId : 0,
+                        (c.getNetwork() != null) ? c.getNetwork().getNetId() : 0,
                         spiRecord.getSpi(),
                         c.getMarkValue(),
                         c.getMarkMask(),