Do not use ListenerRegistration in mdsal-dom-api

Do not use ListenerRegistration in mdsal-dom-api

Using ListenerRegistration implies unneeded generics and details the
clients already know (the instance they registered).

Reduce clutter and verbosity by using a simple Registration return in
all mdsal-dom-api methods.

This cascades to mdsal-dom-broker, and other components, allowing them
to be further simplified.

JIRA: MDSAL-843
Change-Id: I067ae11bff4ce2adac297f7d8caae2d2b695bbfd
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>