[OpenMP][NFC] Merge MemoryManager into PluginInterface (#73678)

Similar to #73677, there is no benefit from keeping MemoryManager
seperate; it's tied into the current design. Except the move I also
replaced the getenv call with our Env handling.

GitOrigin-RevId: 0783bf1cb3e47fc48813ee63f42d72b70d8cb792
8 files changed