Generate bindingToString() and use it in generated implementations

Generate bindingToString() and use it in generated implementations

As we are gearing towards generating default methods in interfaces,
we need the ability to completely analyze all a target type from
a template, not from a generator.

This ability was previously available to Builder*Template, now it
is part of JavaFileTemplate and hence can be picked up by
InterfaceTemplate as needed.

JIRA: MDSAL-479
Change-Id: I1e939aca15f2f77b82a83ca454503522c96bcabe
Signed-off-by: illia.ihushev <illia.ihushev@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>