Audio focus: make media fade out temporary

 This change only affect media/game apps that request and lose focus
with GAIN and LOSS (i.e. no temporary gain/loss).
 The fade out mechanism for media apps losing audio focus intentionally
left players in a muted state at the end of the fade out ramp. If
they were starting again after a focus request, they were unmuted.
 But some  applications don't respect audio focus, e.g. they request focus
with GAIN when they start playing, but keep playing even after they lose
focus with LOSS. Such "offending" apps were left muted as they were not
expected to keep playing.
 This change is a mitigation of the behavior of offending apps: 2s
after an app has been notified it lost focus, if it still had
players, they will be unmuted.
  - for apps that followed the audio focus guidelines, their player
was paused by then, so no change expected
  - for offending apps, their audio will be heard again, shortly
after the new app (the new focus owner) is likely to have started
playing.

Bug: 196186950
Test: atest AudioFocusTest
Change-Id: I873fd1371ae499e50f5e6a60456ce4b0139f2d34