Fix that InputDeviceReaderThread that got killed during startup

InputDeviceReaderThread could be killed if a key or touch event
was received before initiation made by PolicyThread was made. To
solve this, the start call for the InputDeviceReader thread was
delayed until initalization of the PolicyThread was done in
the WindowManagerService.

Change-Id: Ifa7de7ccfadd66ecc2b14c6273e9be32b8e0cb4a