Improve ImsManager efficiency and make threading more obvious

1) Superficial change to classes using ImsManager that register callbacks.
Instead of internally creating Executors on the threads, make it obvious
to the callers by making them pass in an Executor.

Test: atest FrameworksTelephonyTests
Change-Id: Ie393ae6610ebfbd4c939faec7fd8aaf96cbf6683