IpSecService - Convert mNextResourceId from AtomicInt to Int

The mNextResourceId variable is only accessed within synchronized
blocks, so there is no need to use an atomic integer to synchronize
it. This eliminates the misleading notion that the variable is
accessed outside of guarded blocks, which it is not.

Bug: 62279167
Test: cts
Change-Id: I815835622659f54d2d2d33b349b17c632ebced8d