Add RIL_REQUEST_SHUTDOWN to notify RIL that device is shutting down

When device is in airplane mode, SIM card and modem may be powered on.
RIL can use RIL_REQUEST_SHUTDOWN to shutdown the modem and SIM card
gracefully.

Bug: 9773278
Change-Id: I6afa9e810cefc7855f7ae42e51f39cc51fa2aa6e
diff --git a/libril/ril.cpp b/libril/ril.cpp
index 4b3e1b8..feaf60f 100644
--- a/libril/ril.cpp
+++ b/libril/ril.cpp
@@ -4710,6 +4710,7 @@
         case RIL_UNSOL_SRVCC_STATE_NOTIFY: return "UNSOL_SRVCC_STATE_NOTIFY";
         case RIL_UNSOL_HARDWARE_CONFIG_CHANGED: return "HARDWARE_CONFIG_CHANGED";
         case RIL_UNSOL_DC_RT_INFO_CHANGED: return "UNSOL_DC_RT_INFO_CHANGED";
+        case RIL_REQUEST_SHUTDOWN: return "SHUTDOWN";
         default: return "<unknown request>";
     }
 }