[amanKG777]

This PR adds the ability to silence specific sound events in sound themes (both in settings and via theme.json), adds custom accessibility actions to the theme sound customization screen, and introduces a screen_on sound event that defaults to silent in Backtalk.

What was changed

1. Silencing sound events ("No sound")

• In Sound themes > Sounds, selecting an event now includes an option for No sound.
• When an event is set to no sound, its summary shows "No sound" and audio feedback is skipped for that event.
• Silenced events can be reset back to default at any time using "Use Backtalk's sound" (or "Remove the sound" for control sounds).
• In theme.json, sound themes can now silence events by declaring:

  "sounds": {
    "gesture_begin": "none",
    "gesture_end": "none"
  }

  This makes it easy to create sound themes inspired by screen readers like iOS VoiceOver that do not have gesture sounds.

2. Custom accessibility actions in theme sounds

• For screen reader users navigating the sound list, each sound row now provides direct accessibility actions (via the actions rotor / local context menu):
  • Preview (plays preview if sound is audible)
  • Set to no sound (silences the event directly without opening dialogs)
  • Use Backtalk's sound / Remove the sound (resets modified sound back to default)
  • Choose a sound file (opens file picker)

3. Screen on event

• Added screen_on as an individual feedback sound event (in Individual sounds and vibrations and in theme Sounds).
• By default, screen_on plays no sound in Backtalk (same behavior as before).
• Users and sound themes can now assign a custom audio file to play whenever the screen turns on.

4. Tests and Documentation

• Added unit tests in SoundThemeManifestTest, SoundThemesTest, and ThemeFeedbackTest.
• Updated themes.md documenting "sounds" in theme.json and the screen_on event.
• Added Czech translations for the new strings.

Testing

• Unit tests pass with ./gradlew :talkback:testPhoneDebugUnitTest :utils:testPhoneDebugUnitTest.
• Tested on a physical Poco device running Android 16:
  • Silenced gesture start/end and verified gestures no longer play sounds.
  • Used accessibility custom actions to set "No sound" and reset back to default without opening the dialog.
  • Assigned a custom sound file to "Screen on" and verified the sound plays upon turning the screen on, and verified it remains silent when left at default.