Change Nat464Xlat lifecycle.

Currently, NetworkAgentInfo keeps a Nat464Xlat object only on
networks where we're starting clatd (i.e., IPv6-only networks).
Simplify this code by making the Nat464Xlat object final and
always non-null. This allows us to use Nat464Xlat to store
information, such as the NAT64 prefix, even if the clat daemon
has not been started yet.

Also, remove the STOPPING state which waits for the interface to
be removed. Instead, when stop() is called, immediately enter the
IDLE state.
- This is necessary in order to be able to call start() again
  before the interface removal notification has arrived.
- It's also arguably more correct than the current code,
  because when stop() returns clatd has already terminated
  (ClatdController::stopClatd calls waitpid), and thus the tun
  fd is already closed and the tun interface is gone.

Also, now that Nat464Xlat objects are reused after stop(), add
test coverage for calling start() after stop, in both cases:
- The notification that the previous interface was removed
  arrives before the second start().
- The notification that the previous interface was removed
  arrives after the second start() but before the notification
  that the second interface was added.

Also fix a couple of lint warnings.

Test: builds, boots
Test: atest FrameworksNetTests
Test: clatd stops when IPv4 address added
Test: clatd restarts after "adb shell killall clatd"
Change-Id: I3dc66d155aa27606681f3473daf2170434d8c6d0