Add setting for the assist gesture sensitivity control.

The sensitivity control is only visible when the assist gesture itself
is enabled.

Test: make -j RunSettingsRoboTests; manual test on supported/unsupported
      configurations.

Change-Id: I584975794c5cf9d788e93167292d142ae2faf1c5

src/com/android/settings/gestures/AssistGestureSettings.java

@@ -58,6 +58,7 @@ public class AssistGestureSettings extends DashboardFragment {
             Lifecycle lifecycle) {
         final List<PreferenceController> controllers = new ArrayList<>();
         controllers.add(new AssistGesturePreferenceController(context, lifecycle));
+        controllers.add(new AssistGestureSensitivityPreferenceController(context, lifecycle));
         return controllers;
     }