Android - Reading Device Policy and IsActivePasswordSufficient

I want to check that the current device has sufficient security. The DevicePolicyManager and its IsActivePasswordSufficient look like they will do the job just fine.

The docs say that I should request a number of permissions. Which I have done by editing the AndroidManifest.xml file, adding a Permission attribute in AssemblyInfo and setting a MetaData attribute on my Activity.

However every time I call it I get a security Exception,

java.lang.SecurityException: No active admin ComponentInfo{CheckSecurity.CheckSecurity/checksecurity.android.MainActivity}

Does anyone have an experience of this API?

Thanks