Remove old layout and flag

Since the new layout is complete and can handle notification action buttons,
we can remove the old one now.

- Deleted layout files (xml and PlayerViewHolder), and moved
PlayerSessionViewHolder into MediaViewHolder
- Removed the flag MEDIA_SESSION_LAYOUT
- Refactored MediaControlPanel to be more readable

Bug: 218875984
Test: atest com.android.systemui.media
Test: atest QSPanelControllerTest QuickQSPanelControllerTest
Change-Id: I37681542dec5cf10ec7346aa9f888f8f890d2c11