[Thread] clear calling identity before invoking app-supplied callback

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