[csl] use host time for counting CSL synchronization elapsed time

Limit periodic radio API calls to `otPlatRadioGetNow` in order to save power.

The impact of the error in the receive window duration calculation is small
compared with the power consumption savings obtained with the reduction of
radio API calls in serialized platforms.

src/core/mac/sub_mac.cpp

@@ -352,7 +352,7 @@ void SubMac::HandleReceiveDone(RxFrame *aFrame, Error aError)
         // Assuming the risk of the parent missing the Enh-ACK in favor of smaller CSL receive window
         if ((mCslPeriod > 0) && aFrame->mInfo.mRxInfo.mAckedWithSecEnhAck)
         {
-            mCslLastSync = TimeMicro(static_cast<uint32_t>(aFrame->mInfo.mRxInfo.mTimestamp));
+            mCslLastSync = ot::TimerMicro::GetNow();
         }
     }
 #endif // OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
@@ -611,11 +611,11 @@ void SubMac::HandleTransmitDone(TxFrame &aFrame, RxFrame *aAckFrame, Error aErro
             mCallbacks.RecordCcaStatus(ccaSuccess, aFrame.GetChannel());
         }
 #if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
-        // Actual synchronization timestamp should be from the sent frame instead of the current time.
+        // Actual synchronization timestamp should be from the sent frame instead of the current local time.
         // Assuming the error here since it is bounded and has very small effect on the final window duration.
-        if (mCslPeriod > 0)
+        if (aAckFrame != nullptr && aFrame.GetHeaderIe(CslIe::kHeaderIeId) != nullptr)
         {
-            mCslLastSync = TimeMicro(static_cast<uint32_t>(otPlatRadioGetNow(&GetInstance())));
+            mCslLastSync = ot::TimerMicro::GetNow();
         }
 #endif
         break;
@@ -1231,13 +1231,10 @@ void SubMac::HandleCslTimer(void)
 
 void SubMac::GetCslWindowEdges(uint32_t &aAhead, uint32_t &aAfter)
 {
+    uint32_t elapsed    = ot::TimerMicro::GetNow() - mCslLastSync;
     uint32_t semiPeriod = mCslPeriod * kUsPerTenSymbols / 2;
-    uint32_t curTime    = static_cast<uint32_t>(otPlatRadioGetNow(&GetInstance()));
-    uint32_t elapsed;
     uint32_t semiWindow;
 
-    elapsed = curTime - mCslLastSync.GetValue();
-
     semiWindow =
         static_cast<uint32_t>(static_cast<uint64_t>(elapsed) *
                               (Get<Radio>().GetCslAccuracy() + mCslParentAccuracy.GetClockAccuracy()) / 1000000);