overheat: fix re-launching issue and power off issue after 30 sec

overheat: fix re-launching issue and power off issue after 30 sec

- re-launching issue
  = Previously, the overheat popup was re-launched if deviced requests
    to launch the popup again. It looked weird for User.
  = Now, the overheat popup checks the existence of the popup and
    skips re-launching itself.

- power off issue
  = Previously, the overheat popup just hided the popup after 30 seconds.
  = Now, the overheat popup turns off the target after 30 seconds.

Change-Id: Ic078e767effa0f90df1ffd5853e44f00005d54ee
Signed-off-by: taeyoung <ty317.kim@samsung.com>