Wait for handlers to clear in SST test before continuing

SST test was flaky because SST test handler was accessing mocks
while the test thread was modifying them (reproducable by lowering
waitForMs timeout locally).

Instead we will wait for the handler thread to clear by checking to
see if the thread has messages every 100 ms for up to 1 sec.

Bug: 140313454
Test: atest FrameworksTelephonyTests:ServiceStateTrackerTest
Change-Id: I1c6a1763520d4774d20113429eba44f26823f152