gd: Move mgmt files from gd/hal to gd/os

Since mgmt provides a way to interact with linux kernel, move
related files to gd/os. Also spell out Mgmt fully to Management
and create singleton methods.

Bug: 372926952
Test: m -j
Flag: Exempt, no change to default behavior
Change-Id: Ia077af553bcbd15026a275140e4bc86bfbb03a11