Do not send and receive datagrams at the same time.

The following changes are made in this CL:
- DatagramDispatcher:
-- send datagram only if modem is not receiving datagrams.

- DatagramReceiver:
-- Poll for pending datagrams only if modem is not sending datagrams.

Bug: 275670811
Test: atest com.android.internal.telephony.satellite
      atest CtsTelephonyTestCases:android.telephony.satellite.cts
      Flashed build on raven-userdebug: calls and sms are working

Change-Id: I6fcf6709453daa75b69731fb19decd6fe90f4c6d