Fix storing the wrong value of mLockdown in setting

When user is stopped, the Vpn#onUserStopped() will be called and
the value of mLockdown will be set to false then store into
setting.
This is a wrong behavior because user doesn't change it, so for
this kind of case, there is no need to store the value of
mLockdown in setting.
In fact, there is no need to call Vpn#saveAlwaysOnPackage() when
user is stopped because there is nothing changed.

Bug: 168500792
Test: atest FrameworksNetTests
Change-Id: Ie85a347216614b7873bfdf199165d89527ada3a8
(cherry picked from commit 9226fc37)