[SPEAK-ON-MUTE] Update the prefs for system UI.

Update the pref name to a more suitable one. Add logic to disable show
opt-in nudge pref when user has changed the setting.
Add a new pref to count the number of times the opt-in nudge has shown.

Update the unit tests accordingly.

'*CrasAudioHandlerTest.SpeakOnMuteDetectionPrefSwitchTest*'
'*AudioDevicesPrefHandlerTest.ShouldShowSpeakOnMuteOptInNudgePrefRegistered*'

Bug: b/283345021
TEST: chromeos_unittests --gtest_filter=
TEST: chromeos_unittests --gtest_filter=
Change-Id: Id24b66f12409be01cd6a486464eb207bc326ff0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4546447


Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Li-Yu Yu <aaronyu@google.com>
Commit-Queue: CJ Huang <chenjih@google.com>
Reviewed-by: Charles Zhao <charleszhao@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1146715}