[diag] set rx_when_idle mode when diag start/stop (#11035)

This commit fixes an issue related to #9554 where a new radio
capability is introduced: RX_ON_WHEN_IDLE.

In this commit, we set `rx_on_when_idle` to be true when calling `diag
start`, and false when calling `diag stop` for devices which support
`rx_on_when_idle`.

This fix ensures that packets can still be received if `diag` is used
after `ifconfig down`.

src/core/diags/factory_diags.cpp

@@ -495,6 +495,7 @@ Error Diags::ProcessStart(uint8_t aArgsLength, char *aArgs[])
 
     IgnoreError(Get<Radio>().Enable());
     Get<Radio>().SetPromiscuous(true);
+    Get<Mac::SubMac>().SetRxOnWhenIdle(true);
     otPlatAlarmMilliStop(&GetInstance());
     SuccessOrExit(error = Get<Radio>().Receive(mChannel));
     SuccessOrExit(error = Get<Radio>().SetTransmitPower(mTxPower));
@@ -546,6 +547,7 @@ Error Diags::ProcessStop(uint8_t aArgsLength, char *aArgs[])
     otPlatAlarmMilliStop(&GetInstance());
     otPlatDiagModeSet(false);
     Get<Radio>().SetPromiscuous(false);
+    Get<Mac::SubMac>().SetRxOnWhenIdle(false);
 
     Output("received packets: %d\r\nsent packets: %d\r\n"
            "first received packet: rssi=%d, lqi=%d\r\n"