Resolve notification bar issue

[goldy1992]

Requirements

WHEN: player is started
THEN: Notification is not displayed (i.e. playerNotificationManager.setPlayer(null) on startup)

WHEN: a song if played or resumed:
THEN: if notification is not displayed, display it, (i.e. playerNotificationManager.setPlayer(player))

WHEN: media player is playing
THEN: notification is active and cannot be removed

WHEN: media player is paused
THEN: notification can be removed

WHEN: back is pressed on the MainActivity and the app is exited
AND: the player is in ANY state
AND: The notification is present
THEN: The notification is NOT removed

WHEN: The media player is playing and the app is removed from the current tasks
THEN: The notification is NOT removed

WHEN: The media player is paused and the app is removed from the current tasks
THEN: The notification is removed

[goldy1992]

merged to develop