Address review: keep connectivity subscribers on-main and fix handover

- SuggestionActivity's ConnectionChangeEvent subscriber used a bare @subscribe
  (POSTING). Since the event is now posted from a background thread, its
  updateEmptyView() touched Views off the main thread. Pin it to ThreadMode.MAIN.
- NetworkConnectionMonitor.onLost no longer assumes the device is offline: during
  a network handover (e.g. Wi-Fi -> cellular) the replacement is already the
  default, so re-query the active network's capabilities instead of posting a
  spurious disconnected event.

Author: nbradbury

WordPress/src/main/java/org/wordpress/android/networking/NetworkConnectionMonitor.kt

@@ -3,6 +3,7 @@ package org.wordpress.android.networking
 import android.content.Context
 import android.net.ConnectivityManager
 import android.net.Network
+import android.net.NetworkCapabilities
 import android.os.Handler
 import android.os.HandlerThread
 import org.greenrobot.eventbus.EventBus
@@ -44,7 +45,9 @@ class NetworkConnectionMonitor @Inject constructor() {
 
         val callback = object : ConnectivityManager.NetworkCallback() {
             override fun onAvailable(network: Network) = onConnectivityChanged(true)
-            override fun onLost(network: Network) = onConnectivityChanged(false)
+            // onLost fires for the network that was lost, but during a handover (e.g. Wi-Fi -> cellular) the
+            // new default has already arrived via onAvailable, so re-query rather than assuming we're offline.
+            override fun onLost(network: Network) = onConnectivityChanged(hasActiveConnection())
         }
         networkCallback = callback
 
@@ -76,4 +79,15 @@ class NetworkConnectionMonitor @Inject constructor() {
             EventBus.getDefault().post(ConnectionChangeEvent(isConnected))
         }
     }
+
+    /**
+     * Whether there is currently a default network capable of reaching the internet. Used to distinguish a
+     * true disconnection from a handover between networks, where the replacement is already the default.
+     */
+    private fun hasActiveConnection(): Boolean {
+        val manager = connectivityManager ?: return false
+        val activeNetwork = manager.activeNetwork ?: return false
+        val capabilities = manager.getNetworkCapabilities(activeNetwork) ?: return false
+        return capabilities.hasCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET)
+    }
 }

WordPress/src/main/java/org/wordpress/android/ui/suggestion/SuggestionActivity.kt

@@ -12,6 +12,7 @@ import android.view.inputmethod.EditorInfo
 import androidx.activity.addCallback
 import org.greenrobot.eventbus.EventBus
 import org.greenrobot.eventbus.Subscribe
+import org.greenrobot.eventbus.ThreadMode
 import org.wordpress.android.R
 import org.wordpress.android.WordPress
 import org.wordpress.android.databinding.SuggestUsersActivityBinding
@@ -260,7 +261,7 @@ class SuggestionActivity : BaseAppCompatActivity() {
         super.onPause()
     }
 
-    @Subscribe
+    @Subscribe(threadMode = ThreadMode.MAIN)
     fun onEventMainThread(event: ConnectionChangeEvent) {
         viewModel.onConnectionChanged(event)
         updateEmptyView()