Prevent showing immersive confirmation unexpectedly

By design, if the user presses the power key twice within 5 seconds
(panic), it will show immersive confirmation again at the next time
entering immersive mode.

But there is a bug on calculating the time between pressing the power
key. The time get from event.getDownTime() will stop when the device
is sleeping without being charged.

So here uses SystemClock.elapsedRealtime() instead.

https://code.google.com/p/android/issues/detail?id=80772

Change-Id: I4dca9dc260904f1c5b60464834781e2a82a9bcf4