Notification playback: pause on release

When releasing a MediaPlayer outside of a playback
completion (back to back notifications), pause and
wait 100ms before releasing the player.

Bug: 193832902
Test: create back-to-back notifications, check for glitch
Change-Id: Ib8baba4bf2b41e75b526e8490b6c2f31c85c193c
(cherry picked from commit c69743ea)