Move to Excutor interface to spawn threads for testing

When creating a new thread in ImsManager to perform
ImsConfig updates, wrap creating the new thread in an
executor so it can be overriden to be single threaded
when testing.

Bug: 123641963
Test: atest FrameworksTelephonyTests:ImsManagerTest
Change-Id: Ic3d5d2f18a91903771f8397f78a8eafdf53889de