Tapping music (media) player notification should open the player

# Note: Items 1 & 2 are addressed; leaving open to track enhancement idea in item 3 and updated issue title to reflect that idea

PROBLEM 1 - music controls not working

### ~~Steps to reproduce~~
1. ~~Open music file~~
2. ~~try to pause it~~

~~Screenshot: music controls not responsive.~~


PROBLEM 2 - exiting & re-opening music player renders weird layout

### ~~Steps to reproduce~~
~~1. Open music file~~
~~2. hit HOME to minimize it~~
~~3. open it again from multitasking view~~

~~A non-responsive layout is rendered with no music controls, FAB and some elements from main view:~~


PROBLEM 3 - tapping music player notification produces no response
### Steps to reproduce
1. play music
2. hit HOME to return to homescreen
3. tap nextcloud music player notification

Nothing happens. I believe the player should open.

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Android version: 10

Device model: OP5T

Nextcloud app version: 3.13.0

Nextcloud server version: 19