Fix issue 2459650.

This change fixes a problem where an unwanted tone is generated by audio policy manager when a MT call is answered.
This is because of a policy that replaces high visibility system sounds (ringtones, alarms...) by a beep when in call.
There is a transitory phase while the call is being answered where the phone state is changed to IN_CALL but the
ringtone is still playing. The audio policy manager then mutes the end of the ringtone and starts playing a beep
in replacement because the ringtone is categorized as high visibility.

The fix consists in changing the ringtone stream type from high visibility to low visibility. This is not a problem as
the only actual use case where a ringtone would be generated while in call is if another call is received.
But in this case, the phone system does not generate a ringtone but a call waiting tone instead.
It is therefore not required to handle a ringtone as a high visibiltiy tone that must be somehow signaled to the user
while in call.

media/libmedia/AudioSystem.cpp

@@ -694,7 +694,9 @@ bool AudioSystem::isBluetoothScoDevice(audio_devices device)
 
 bool AudioSystem::isLowVisibility(stream_type stream)
 {
-    if (stream == AudioSystem::SYSTEM || stream == AudioSystem::NOTIFICATION) {
+    if (stream == AudioSystem::SYSTEM ||
+        stream == AudioSystem::NOTIFICATION ||
+        stream == AudioSystem::RING) {
         return true;
     } else {
         return false;