Remove static memoization from AudioPlayerStateMonitor.getInstance

This static memoization can result in the Context parameter being
ignored, and an instance associated with one Context being returned
to a caller passing a different Context.

The static memoization was already present before the Context parameter
was introduced in ag/6935293.

Bug: 438917725
Test: Presubmit
Flag: com.android.media.mediasession.flags.remove_audioplayerstatemonitor_static_memoization
Change-Id: Ic8ead19edb46ae0f65b6664a2b46b30b0aca29f4