Use CallableItem in place of MethodItem

The `MethodItem` type represents both a normal method as well as being
the super type of `ConstructorItem`. That organization causes a number
of issues in Metalava and has lead to quite a few issues where code
that only expected to be run on methods and not constructors was run on
both. So, a new type `CallableItem` has been added that will eventually
become the super type of both `MethodItem` and `ConstructorItem` and
`ConstructorItem` will no longer extend `MethodItem`.

This change prepares this code for that change.

Bug: 352481366
Test: ./gradlew
Change-Id: I9f2f60ce60337db55246a43e23285921a895519b