Add feature flags for VolumePlus

BUG: 309052662
Test: m Settings
Change-Id: I36aad5bb290aee835d586f0791347ab3a05e77f1

aconfig/settings_panel_flag_declarations.aconfig

@@ -0,0 +1,8 @@
+package: "com.android.settings.flags"
+
+flag {
+  name: "enable_volume_plus_quick_settings"
+  namespace: "pixel_cross_device_control"
+  description: "Gates whether to enable VolumePlus quick settings panel."
+  bug: "309052662"
+}