[Network Connection] Start dialog as affinity activity

Use affinity to keep a clear back stack to prevent caller activity from
stopped stage. If there is only NetworkRequestDialog activity in back stack,
caller will not be covered and then only become paused stage.

Bug: 128995251
Test: manual
Change-Id: I552b942810c919a41a690beff523a01aba1e4d0b

AndroidManifest.xml

@@ -2622,6 +2622,8 @@
             android:name=".wifi.NetworkRequestDialogActivity"
             android:theme="@style/Transparent"
             android:excludeFromRecents="true"
+            android:launchMode="singleTop"
+            android:taskAffinity=".wifi.NetworkRequestDialogActivity"
             android:exported="true"
             android:permission="android.permission.NETWORK_SETTINGS">
             <intent-filter>