SystemUI: Don't append app name to file on lockscreen

When device is locked, current implementation will append the current
app name to file when taking screenshot. This causes information leaks.

To prevent this, we check for isKeyGuardLocked() and fall back not to
append the app name when taking screenshot on lockscreen.

Change-Id: I04498faf51986e1a3723a32befd97d29e1f3fe58