Android Things relies heavily on Android libraries to offer flexibility and functionality. Given that Android Things is optimized for embedded devices that may not contain the same feature set as an Android phone or tablet, not all the APIs are present. In most cases this is coming from the restriction of not requiring a UI. There is a reference list of system APIs and Google APIs that are supported and unsupported in the Appendix.
You can use hasSystemFeature() to determine whether a given device feature is supported at runtime.
As a rule of thumb, if it requires a system service -such as a calendar or telephone- or a strong UI component -such as AdMob or Sign-in- it is likely to not be supported, but the number of available services is remarkably large.