Set mCurrentActivePosition in one place.

mCurrentActivePosition meant to prevent rerendering the whole device
list when only one route has changed. Unfortunately, it doesn't work
properly when there are more than one active devices e.g. group
playback.

Flag: EXEMPT refactor
Bug: 387570618
Test: atest SystemUiRoboTests:MediaOutputAdapterTest
Change-Id: Ia4a0eeb47aa04e0ec1667c060849282da91214d0