HeadsetStateMachineTest: wait for handler to quit

Or else mock are reset too quickly and the thread crash
This is due to the test not controlling its execution

Bug: 398953257
Fix: 398953257
Test: atest BluetoothJavaUnitTests
Flag: TEST_ONLY
Change-Id: Ice7333a249341104b3a0d14252c17b939a46964d