Revert "Selectively show zen footer."

While playing music, if zen mode is on, the user must exit it via the
quick settings tile or via the settings menu, even if they used the
volume buttons to enter the state. We shouldn't have such a restriction.

Ticket: CYNGNOS-2662

This reverts commit 97eff878.

Change-Id: I305d94e5b187db6fa01e5453bcdadbd1687cf43f
(cherry picked from commit 31f30262)