init: add exec_background command

This command functions similarly to `exec` except that it does not
cause init to halt executing commands until the process has
terminated.  It is useful for launching simple one time background
tasks.

Bug: 65736247
Test: create an exec_background service and see it function properly
Change-Id: I719c8b85479b65201770aedc0a13191303007c11
2 files changed