Add a delay before showing the TV power off toast

The current toast is shown right after the power button is pressed. In some cases, the toast show up before enter dreaming and made bad user experience. This can be mitigated by delaying it a little bit.

A new integer config parameter to tweak this delay for different devices.

Bug: 434774888
Test: manual on device
Flag: NONE small change for TV only

Change-Id: I22b1569a71ff087b31455949749b5be946f27a94