LockScreenActivity is start when entering the app after the task was removed from Recent Activity
Let assume the passphase expiration interval was set 10 minutes and exit the app.
If we keep the task in background in less than 10 minutes, the LockScreenActivity won't be started. But if we remove it from Recent Activity,
LockScreenActivity will be started when entering the app after the task was removed from Recent Activity.
This is somehow not some users' expectation. If we take TextSecure for instance, even if we manually remove the task from Recent Activity, the Secret will still be kept and LockScreenActivity won't be triggered until the passphase expires.