services: Add hook to run device specific shutdown code.

Add hook to call OEM specific shutdown synchronously. OEMs
need to add ShutdownOem class and define deviceRebootOrShutdown
function in that.

ShutdownThread class would call this OEM defined function
just before shutting down the device.

Change-Id: I620495bfc99c6b0855a98ac54c6ede96e20ff1b8