Prevent scanning during DHCP process

Wi-Fi should be in active state during the entire DHCP process, and
shouldn't go to IEEE 802.11 power save mode. If the framework requests
scan during the DHCP process, the Wi-Fi chip has to start scanning
on channels different from the current one, and going to power save
mode is a prerequisite for scan. The result directly impacts user
experience: DHCP process takes longer, and even can fail.

Change-Id: I8171388bb70072e4c42cb3c074dd955da84e494b

wifi/java/android/net/wifi/WifiStateMachine.java

@@ -3412,6 +3412,10 @@ public class WifiStateMachine extends StateMachine {
               case CMD_SET_HIGH_PERF_MODE:
                   deferMessage(message);
                   break;
+                  /* Defer scan request since we should not switch to other channels at DHCP */
+              case CMD_START_SCAN:
+                  deferMessage(message);
+                  break;
               default:
                   return NOT_HANDLED;
           }