Make CallService sounds thread-safe

[liviu-timar]

🎯 Goal

Make sound-related operations thread safe in CallService to avoid race conditions related to object instantiation, playing/stopping sounds and cleaning resources.

🛠 Implementation details

• Synchronized mediaPlayer and ringtone object creation.
• Synchronized property and method access for these objects
• Marking object references with @Volatile is not needed, as both the instantiation and subsequent operations are synchronized, which takes care of visibility across threads.