SystemUI: Fix system ui crash while continuously click flash light tile issue

In FlashlightController, after set mSession/mSurface null in teardown(),
mSession is reassigned in callback of mSessionListener.onConfigured().
This will lead to no Surface object for mSession.

Don't reassigned mSession when mSurface is already set null.

CRs-Fixed: 750124

Change-Id: I1c4214cc4b58375e82740853c5aff9b68530f816