Refactor the Nat464Xlat function for simplicity.

This makes the code easier to understand by making state
transitions more explicit. It also makes it easier to address a
TODO to turn the class into a StateMachine.

This should be an exact no-op refactoring. The current cases
covered by the code (all mutually exclusive) are:

1. requiresClat && !isPrefixDiscoveryStarted
   Action: startPrefixDiscovery()
   Equivalent to IDLE && requiresClat, because
   isPrefixDiscoveryStarted returns true for every state except
   IDLE.

2. requiresClat && isPrefixDiscoveryStarted && shouldStartClat
   Action: start()
   Equivalent to DISCOVERING && shouldStartClat,
   because isPrefixDiscoveryStarted is true in DISCOVERING,
   STARTING, and RUNNING, but start() does nothing if mState is
   STARTING or RUNNING.

3. requiresClat && isPrefixDiscoveryStarted && !shouldStartClat
   Action: stop()
   Equivalent to (STARTING or RUNNING) && !shouldStartClat,
   because isPrefixDiscoveryStarted is true in DISCOVERING,
   STARTING, and RUNNING, but stop() does nothing if mState is
   not STARTING or RUNNING.

4. !requiresClat && isStarted
   Action: stop()
   Equivalent to (STARTING or RUNNING) && !requiresClat,
   because isStarted() is only true in STARTING and RUNNING.

5. !requiresClat && !isStarted && isPrefixDiscoveryStarted
   Action: leaveStartedState()
   Equivalent to DISCOVERING && !requiresClat, because
   the only state with isPrefixDiscoveryStarted and !isStarted
   is DISCOVERING.

Also, simplify case #5. In this case, calling leaveStartedState
is superfluous, because in the DISCOVERING state:
- There is no need to call unregisterObserver, since the observer
  is only registered when entering STARTING and is unregistered
  when going back to DISCOVERING or IDLE.
- mIface and mBaseIface don't need to be set to null because they
  are only set to non-null when entering STARTING and nulled out
  when going back to DISCOVERING or IDLE.

Bug: 126113090
Bug: 150648313
Test: covered by existing ConnectivityServiceTest and Nat464XlatTest
Change-Id: Ice536bcb269cc8b040c6e7a72c15d0bc8b5bd235