base-bearer: make sure 'ReloadStatsSupported' is set before transitioning to 'Connected'

By the time we report the bearer as 'Connected', the DBus client
should also be able to know right away the value of
'ReloadStatsSupported', instead of needing to wait for it to be
updated asynchronously.

This logic adds one additional step in the generic connection
sequence for this purpose, which is run after the modem is connected
but before we report the state change via DBus.

introspection/org.freedesktop.ModemManager1.Bearer.xml

@@ -381,7 +381,15 @@
     <!--
         ReloadStatsSupported:
 
-        Indicates whether or not the bearer support getting rx and tx bytes statistics.
+        Indicates whether reloading ongoing statistics is supported or not.
+
+        This property applies exclusively to the statistics that are queried from
+        the modem periodically; i.e. "rx-bytes", "tx-bytes", "uplink-speed" and
+        "downlink-speed".
+
+        The property is initialized to a fixed value as soon as the first
+        connection attempt has successfully finished. Reading this value before
+        the first successful connection attempt will always report %FALSE.
 
         Since: 1.20
     -->