Adds the Thread service unit test skeleton and make the services
testable.
Initially, a few trivial unit tests for existing APIs are added in
ThreadNetworkControllerServiceTest.java, and more tests will be added in
separate CLs later.
Bug: 317555104
Change-Id: I7b9e5f3663493d8da2448568de792e4272ee1df6
Per API council feedback and API guideline at
go/android-api-guidelines#provide-executor, it should call
Binder.clearCallingIdentity() before invoking the app-supplied callbacks
so that the app doesn't use the system-server's identity for permission
check in the callback.
Bug: 309952666
Test: atest ThreadNetworkUnitTests
Change-Id: I3cfbce29c0a516506bb76a44b151a583c8477806