Simplify implementation of isEnabled

Implement in terms of getState()

Future CL will remove this API from the AIDL.

Reimplement test methods in terms of getState,
and remove references in other tests as they don't
actually reference isEnabled under the hood.

Bug: 45171640
Test: compile
Change-Id: I73ebecb759493515f25c510208c3f4ab5b93d3a4