[VpnSettings]Crash when activity has been recycled

This is a case we found in monkey test, the main thread handles message while the activity has been recycled. It will raise a NullPointerException when we use getActivity() in following lines.
Assumpt the situation is: settings run in background and activity been recycled quickly, but handler still in the process.

Test: NA

Change-Id: I19db4f13e13294618ec8e42e4d9c54ce23504344
Signed-off-by: liurong <liurong@xiaomi.com>