Send shutdown command to modem

- The shutdown command is not sent to
the modem for a graceful shutdown when
the modem is in offline state in which,
the RADIO_POWER off request is rejected
- Send the shutdown command as soon as
the device has been given a chance to
power down the radio

CRs-Fixed: 977534
Change-Id: I2fc2eae04918f3927e93924f6ae7b3e19f920de2