De-Flakify ImsSmsDispatcherTests

Remove the separate handler thread from the unit tests,
since the tests do not currently require it and *hopefully*
remove any possiblity for this test to have synchronization problems.

Bug: 116754536
Test: atest FrameworksTelephonyTests
Change-Id: I942adb69a1a2424cbffb1e0962b71f6f2437c6e1