Match players based on notification key

One player per package caused problems with RCN because all RCN
notifications come from the same package. We can't remove this step
completely because then every time the user restarts media it would
create a new set of controls.

Instead, match based on the notification key. This will allow one player
per notification tag + ID, so RCN can use different IDs per device to have
multiple players visible at the same time.
This is consistent with how NotificationManager identifies notifications
to update.

Fixes: 153236983
Test: manual - used cast test apk and cast to multiple devices on a
different phone, observed multiple players

Change-Id: Iaa98b4180725209982beb079d96bfedaa9742b58