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