User action should only stop lock task mode if it is in pinned mode

There are two lock task modes: pinned and locked.
Pinned mode is the one that one that need users action to accept the pinned
request. For locked mode, it is triggered by DPM.
We should not allow user to stop locked mode by user action.
Please notice that it does not happen before because the action to dismiss
lock task mode is pressing both back and recents button and recents button
is hidden in locked mode. But it becomes a problem after the user action
is changed to long pressing back button.

So changes are as follows:
1. Only stop lock task mode if it is pinned mode in
   stopLockTaskModeOnCurrent
2. Remove the permission checking in stopLockTaskModeOnCurrent. I can't
   see the reason we are having this because there is no permission
   checking to in stopLockTaskMode when it is in pinned mode. So you
   can always stop pinned lock task mode anyway by calling
   stopLockTaskMode directly.

Bug: 28184751
Change-Id: I610cc1dfade7737e0b08fc9a13dad29e62e09a32