Fix subscription liveness timeout computation to include MRP backoff.

Right now the amount of time we allow for receiving a subscription report, after
the max-interval has elapsed, is computed by just multiplying our active
liveness timeout by the number of MRP retries.  For a typical always-active
client (hence 300ms active interval) that means 1500ms.

But an actual sender would do backoff in between the MRP messages, so would not
actually give up within 1500ms.  We should be taking that backoff into account
when computing the time we allow to receive the message.

src/app/ReadClient.cpp

@@ -31,6 +31,7 @@
 #include <lib/core/TLVTypes.h>
 #include <lib/support/FibonacciUtils.h>
 #include <messaging/ReliableMessageMgr.h>
+#include <messaging/ReliableMessageProtocolConfig.h>
 
 namespace chip {
 namespace app {
@@ -808,11 +809,17 @@ CHIP_ERROR ReadClient::RefreshLivenessCheckTimer()
         // computing the Ack timeout from the publisher for the ReportData message being sent to us using our IDLE interval as the
         // basis for that computation.
         //
+        // Make sure to use the retransmission computation that includes backoff.  For purposes of that computation, treat us as
+        // active now (since we are right now sending/receiving messages), and use the default "how long are we guaranteed to stay
+        // active" threshold for now.
+        //
         // TODO: We need to find a good home for this logic that will correctly compute this based on transport. For now, this will
         // suffice since we don't use TCP as a transport currently and subscriptions over BLE aren't really a thing.
         //
+        const auto & ourMrpConfig = GetDefaultMRPConfig();
         auto publisherTransmissionTimeout =
-            GetLocalMRPConfig().ValueOr(GetDefaultMRPConfig()).mIdleRetransTimeout * (CHIP_CONFIG_RMP_DEFAULT_MAX_RETRANS + 1);
+            GetRetransmissionTimeout(ourMrpConfig.mActiveRetransTimeout, ourMrpConfig.mIdleRetransTimeout,
+                                     System::SystemClock().GetMonotonicTimestamp(), Transport::kMinActiveTime);
         timeout = System::Clock::Seconds16(mMaxInterval) + publisherTransmissionTimeout;
     }