Invoke shutdownMobileRadios Telephony APIs from ShutdownThread

shutdownMobileRadios API shuts down multiple Radios on MSIM devices and
also gracefully shuts down Radios and SIM cards on single SIM and
MSIM devices. Use this API instead of the old setRadio APIs.

Add RIL_REQUEST_SHUTDOWN to RILConstants. Message is sent to notify
RIL that the device is shutting down.

Bug: 9773278
Change-Id: I6afa9e810cefc7855f7ae42e51f39cc51fa2aa6e