Split core_minimal in two.

Add media_system and media_vendor makefiles which
together make up core_minimal. Document what belongs
in these files.

Also update mainline_system to inherit media_system.

Bug: 80410283
Test: lunch mainline_arm64; m nothing
Change-Id: I030bb6805bf9ff8971ad2e4ba75fc74ace02124b
Merged-In: I030bb6805bf9ff8971ad2e4ba75fc74ace02124b