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
diff --git a/libcutils/Android.bp b/libcutils/Android.bp
index 6d00dc6..dd46750 100644
--- a/libcutils/Android.bp
+++ b/libcutils/Android.bp
@@ -164,6 +164,7 @@
 
     shared_libs: ["liblog"],
     header_libs: [
+        "libbase_headers",
         "libcutils_headers",
         "libutils_headers",
     ],