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