Modify ImsManager to use single thread for testing

ImsManager was starting new Threads to do notify the
modem of ImsConfig changes. This was causing the tests
to be flaky. Make the tests single threaded.

Bug: 123641963
Test: atest FrameworksTelephonyTests:ImsManagerTest
Change-Id: I8b1dc25bb5cc281b27fc0cc342b36692c86da615