Extract logic in notifyWiredAccessoryChanged to make it testable

Extract the calculation logic so that newHeadsetState is returned from a
function. Then we can test the change in between with pre-defined set of
events.

Bug: 411037918
Test: Plug in HDMI and 3.5mm headphone and verify both audio devices exist
Flag: EXEMPT Strict mechanical refactors
Change-Id: Ib1fd68a6ab27705fdaed80529bbe802136ecee79