Rewrite get_sched_policy_name for safety.

This way you'll get a build time error if you make the usual mistake of
adding to the enum but not adding an entry to the array.

Also improve the unit tests, and fix get_sched_policy_name's incorrect
behavior on invalid inputs.

Bug: N/A
Test: ran tests
Change-Id: Iefcb1ec9ef66267837da7a576c8be3d0cfb16cd0