[Thread] add Thread toggle in settings

Allows a user to disable or enable Thread network / radio from
settings.

In this commit, a toggle is added to Settings > Connected devices >
Connection preferences to control Thread state. See the screenshots
below:
1. Thread is on: https://screenshot.googleplex.com/7FqqbzRX6rGwvSb
2. Thread is off: https://screenshot.googleplex.com/AmfRAhzuukULAAG
3. Thread is disabled by airplane mode: https://screenshot.googleplex.com/7zcesyomrveCqFE
4. Thread is searchable: https://screenshot.googleplex.com/9yFL2jeSuEhJwrS

Requirements:
1. the in-take bug: b/327098435
2. See the screenshot above
3. Test with `atest SettingsUnitTests` and manual tests
4. +2 from Yuwen
5. Flagged by "com.android.net.thread.platform.flags.Flags.thread_enabled_platform"
6. It doesn't need B&R, no preferences are added (the state is in
   Android framework (mainline module))
7. Confirmed searchable
8. Code written in Kotlin

Bug: 296990038
Bug: 319077562
Test: atest SettingsUnitTests
Change-Id: Ifa2264b8e59a5112290fd0224cb75ad228732077