Convert the fail cause to DisconnectCause.POWER_OFF

DisconnectCause.POWER_OFF is notified to TeleService when the call
fails due to radio is off. But it will not convert in case of RADIO_OFF

To fix this, add a case that converts the fail cause to
DisconnectCause.POWER_OFF.

Bug: 178435921
Test: Ensure that the reason code is correctly converted to POWER_OFF
      after manually triggering the modem SSR
Change-Id: Ic5fa3e120a1fa395ac8048df4de92446e7ea8a8a