assistant: convert volume to 0.0 - 1.0.

Ash uses 0 - 100 while assistant uses 0.0 - 1.0 for volume. We convert
it in assistant platform volume control impl.

Bug: b/111768214
Test: None
Change-Id: I5e01c1512065e5711f7463662e4f286068599b65
Reviewed-on: https://chromium-review.googlesource.com/1166449Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Muyuan Li <muyuanli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581680}

chromeos/services/assistant/platform/audio_output_provider_impl.cc

@@ -137,16 +137,16 @@ void VolumeControlImpl::SetAudioFocus(
     assistant_client::OutputStreamType focused_stream) {}
 
 float VolumeControlImpl::GetSystemVolume() {
-  return volume_;
+  return volume_ * 1.0 / 100.0;
 }
 
 void VolumeControlImpl::SetSystemVolume(float new_volume, bool user_initiated) {
-  volume_control_ptr_->SetVolume(new_volume, user_initiated);
+  volume_control_ptr_->SetVolume(new_volume * 100.0, user_initiated);
 }
 
 float VolumeControlImpl::GetAlarmVolume() {
   // TODO(muyuanli): implement.
-  return 100.0f;
+  return 1.0f;
 }
 
 void VolumeControlImpl::SetAlarmVolume(float new_volume, bool user_initiated) {