[csl] restart CSL timer when update last sync timestamp (#11601)

This commit involves a small enhancement for CSL to optimize for power
consumption after receiving a frame.

Original implementation: 
- schedule next CSL window during current CSL timer handle
- even if a frame is received, the scheduled CSL timer is not reset
- SSED wakes up much earlier than required if elapsed time is large

Suggested implementation:
- recalculate CSL timer during `UpdateCslLastSyncTimestamp` if it is
  currently running
- SSED can use updated mCslLastSync so that it does not wake up much
  earlier after receiving a frame

src/core/mac/sub_mac.hpp

@@ -501,6 +501,7 @@ public:
 private:
 #if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
     void        CslInit(void);
+    void        RestartCslTimerAfterSyncUpdate(void);
     void        UpdateCslLastSyncTimestamp(TxFrame &aFrame, RxFrame *aAckFrame);
     void        UpdateCslLastSyncTimestamp(RxFrame *aFrame, Error aError);
     static void HandleCslTimer(Timer &aTimer);

src/core/mac/sub_mac_csl_receiver.cpp

@@ -54,6 +54,25 @@ void SubMac::CslInit(void)
     mCslTimer.Stop();
 }
 
+void SubMac::RestartCslTimerAfterSyncUpdate(void)
+{
+    // Only applies for the case where radio supports receive timing.
+    if (RadioSupportsReceiveTiming() && mCslTimer.IsRunning())
+    {
+        uint32_t periodUs = mCslPeriod * kUsPerTenSymbols;
+
+        mCslTimer.Stop();
+
+        // Rewind sample times by one period. HandleCslTimer() will add this
+        // period back, effectively re-evaluating the current CSL period's
+        // schedule using the updated mCslLastSync.
+        mCslSampleTimeRadio -= periodUs;
+        mCslSampleTimeLocal -= periodUs;
+
+        HandleCslTimer();
+    }
+}
+
 void SubMac::UpdateCslLastSyncTimestamp(TxFrame &aFrame, RxFrame *aAckFrame)
 {
     // Actual synchronization timestamp should be from the sent frame instead of the current time.
@@ -62,6 +81,8 @@ void SubMac::UpdateCslLastSyncTimestamp(TxFrame &aFrame, RxFrame *aAckFrame)
     {
         mCslLastSync = TimeMicro(GetLocalTime());
     }
+
+    RestartCslTimerAfterSyncUpdate();
 }
 
 void SubMac::UpdateCslLastSyncTimestamp(RxFrame *aFrame, Error aError)
@@ -82,6 +103,8 @@ void SubMac::UpdateCslLastSyncTimestamp(RxFrame *aFrame, Error aError)
 #endif
     }
 
+    RestartCslTimerAfterSyncUpdate();
+
 exit:
     return;
 }