Fix SystemSpeechSynthesizer related main thread hang on NavigationViewController creation

[azarovalex]

Description

There is an old iOS issue with AVSpeechSynthesizer: each access to it triggers XPC communication, which hangs the thread on which the object is accessed: https://stackoverflow.com/questions/53912072/avspeechsynthesizer-freezes-ios-app-for-a-few-seconds.

Xcode 15.3 now highlights this issue with console warnings like:

Warning example

Thread Performance Checker: Thread running at User-interactive quality-of-service class waiting on a lower QoS thread running at Default quality-of-service class. Investigate ways to avoid priority inversions
PID: 13043, TID: 7057855
Backtrace
=================================================================
3   Foundation                          0x000000018888e37c 5623831D-5719-33A9-9691-759F47D714D1 + 7627644
4   Foundation                          0x00000001882426e4 5623831D-5719-33A9-9691-759F47D714D1 + 1025764
5   Foundation                          0x000000018826974c 5623831D-5719-33A9-9691-759F47D714D1 + 1185612
6   TextToSpeech                        0x00000001a9c78878 TTSSpeechTransformTextWithLanguageAndVoiceIdentifier + 5132
7   TextToSpeech                        0x00000001a9c7df20 AXAVSpeechSynthesisVoiceFromTTSSpeechVoice + 13132
8   TextToSpeech                        0x00000001a9c76020 _TTSIdentifierForVoiceInformation + 17508
9   MyAwesomeApp                  0x00000001051f78f4 $s16MapboxNavigation23SystemSpeechSynthesizerC10isSpeakingSbvg + 44
10  MyAwesomeApp                  0x00000001051f728c $s16MapboxNavigation23SystemSpeechSynthesizerC5mutedSbvW + 60
11  MyAwesomeApp                  0x00000001051f73a8 $s16MapboxNavigation23SystemSpeechSynthesizerC5mutedSbvs + 120
12  MyAwesomeApp                  0x00000001051f9ef4 $s16MapboxNavigation23SystemSpeechSynthesizerCAA0D12SynthesizingA2aDP5mutedSbvsTW + 40
13  MyAwesomeApp                  0x0000000105088934 $s16MapboxNavigation28MultiplexedSpeechSynthesizerC9applyMute33_C78A32105F77292EE97E3E9C94F947E2LLyyFyAA0D12Synthesizing_pXEfU_ + 128

While we can't fix this problem in Apple system library, we only access AVSpeechSynthesizer when it's really necessary. Right now NavigationViewController.viewDidLoad leads to a mutation of SystemSpeechSynthesizer.muted property, which in turn accesses AVSpeechSynthesizer, even though it's not really used.

Implementation

When mutating SystemSpeechSynthesizer.muted, we only try to interrupt it when muted == true, skipping isSpeaking getter.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.54%. Comparing base (dc90edc) to head (fcbd0d1).

@@           Coverage Diff           @@
##             main    #4617   +/-   ##
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  Coverage   60.54%   60.54%           
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  Files         189      189           
  Lines       21297    21297           
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  Hits        12894    12894           
  Misses       8403     8403           

Files	Coverage Δ
...ces/MapboxNavigation/SystemSpeechSynthesizer.swift	46.04% <100.00%> (ø)