[Thread] update Thread settings screen

Per b/327098435 the new Thread settings design proposed
(go/android-thread) is approved.

As a summary, this commit adds a new "connected devices > connection
preference -> Thread" list item and decidated config page for Thread.
Also, we simplified the airplane mode to delegate it to the mainline
code to handle it

Bug: 327098435
Test: atest SettingsUnitTests
Merged-In: Iffbb2471f5a28ec57d30a378f22642fe6ac0b9cc
Change-Id: Iffbb2471f5a28ec57d30a378f22642fe6ac0b9cc