Allow new Now Playing List fetches to happen by cancelling the old request

Problem: The Now Playing List is invalidated when the addressed player
changes. This can happen on initial connection, when the Available Players
change and a player ID is reused, or directly as a result of an Addressed
Player Changed event. Typically, both events come in one after the other,
causing one Now Playing list fetch, immediately followed by another. As
it stands, the first request always finishes, and the future ones are
ignored, despite the list getting cleared/uncached part-way through.
This leads to missing data and/or incorrect lists, which can have
downstream impact on playing items from the Now Playing List as well.

Solution: Let another more recent fetch request for the Now Playing List
invalidate the previous requsst and cause the download process to
restart. These downloads are typically quick and the wasted cycles
downloading sometimes-old-something-invalid data are small, or
necessary.

Tag: #stability
Bug: 177004421
Test: atest AvrcpControllerStateMachine.java
Change-Id: Idcc0a21c966622bfd442ae3b2c7cf1d8f06e75d3