audio: VolumeDialog: Hide ringer button in single volume mode

While in singleVolume mode, the ringer mode can't be changed [1]. But
the VolumeDialog is still showing it, and it doesn't work.

[1] https://cs.android.com/android/platform/superproject/main/+/main:frameworks/base/services/core/java/com/android/server/audio/AudioService.java;l=5687;drc=e7e6ec868a3277517795e77976186fc0dfed9365

Bug: 374870615
Test: press volume keys, see the ringer mode is not shown
Flag: com.android.systemui.hide_ringer_button_in_single_volume_mode

Change-Id: I30508096593084f0e036c2c800c67b6db953a8d1
diff --git a/packages/SystemUI/aconfig/systemui.aconfig b/packages/SystemUI/aconfig/systemui.aconfig
index a984b7d..1c29db1 100644
--- a/packages/SystemUI/aconfig/systemui.aconfig
+++ b/packages/SystemUI/aconfig/systemui.aconfig
@@ -1559,6 +1559,16 @@
 }
 
 flag {
+   name: "hide_ringer_button_in_single_volume_mode"
+   namespace: "systemui"
+   description: "When the device is in single volume mode, hide the ringer button because it doesn't work"
+   bug: "374870615"
+   metadata {
+       purpose: PURPOSE_BUGFIX
+   }
+}
+
+flag {
     name: "qs_tile_detailed_view"
     namespace: "systemui"
     description: "Enables the tile detailed view UI."