Add support for scan always available mode

Modify WifiService to add a controller to track the various
desired states and let the WifiStatemachine actually control
the bring up.

Bug: 8141918

Change-Id: I6e98fd5a29b43c3c50c315eff5255cd0a3eaebcd

core/java/android/provider/Settings.java

@@ -4783,6 +4783,13 @@ public final class Settings {
         */
        public static final String WIFI_ON = "wifi_on";
 
+       /**
+        * Setting to allow scans to be enabled even wifi is turned off for connectivity.
+        * @hide
+        */
+       public static final String WIFI_SCAN_ALWAYS_AVAILABLE =
+                "wifi_scan_always_enabled";
+
        /**
         * Used to save the Wifi_ON state prior to tethering.
         * This state will be checked to restore Wifi after
@@ -5345,6 +5352,7 @@ public final class Settings {
             WIFI_NETWORKS_AVAILABLE_NOTIFICATION_ON,
             WIFI_NETWORKS_AVAILABLE_REPEAT_DELAY,
             WIFI_WATCHDOG_POOR_NETWORK_TEST_ENABLED,
+            WIFI_SCAN_ALWAYS_AVAILABLE,
             WIFI_NUM_OPEN_NETWORKS_KEPT,
             EMERGENCY_TONE,
             CALL_AUTO_RETRY,