On some devices, pausing+resuming (or seeking) would restart playback
from the beginning, but show a progress bar further up. This seems to
have been caused by same race condition-y thing where we were seeking
multiple times in rapid succession. Now we'll only play once, and things
seem to be fine now.
Also removed usage of some deprecated methods.
Fixes#8432
There are several (popular) phone models out there that have bugs in
their MediaPlayer implementation that cause them to be unable to play
voice notes. By moving to ExoPlayer, an application-level media player,
we should avoid most of these headaches and stardardize playback.
Fixes#7748
