unify activity detection in PhoneAPI, turn off BLE API while serial API in use

src/mesh/PhoneAPI.h

@@ -50,6 +50,11 @@ class PhoneAPI
     /// Use to ensure that clients don't get confused about old messages from the radio
     uint32_t config_nonce = 0;
 
+    /** the last msec we heard from the client on the other side of this link */
+    uint32_t lastContactMsec = 0;
+
+    bool isConnected = false;
+
   public:
     PhoneAPI();
 
@@ -85,6 +90,12 @@ class PhoneAPI
     /// Our fromradio packet while it is being assembled
     FromRadio fromRadioScratch;
 
+    /// Hookable to find out when connection changes
+    virtual void onConnectionChanged(bool connected) {}
+
+  /// If we haven't heard from the other side in a while then say not connected
+    void checkConnectionTimeout();
+
     /**
      * Subclasses can use this as a hook to provide custom notifications for their transport (i.e. bluetooth notifies)
      */