Limit trigger terms to shared terms in arrays (#4932)

Limit trigger terms to shared terms in arrays (#4932)

There is a non-standard use of trigger terms in the array theory via `EqualityEngine::addTriggerTerm`.

Trigger terms are only supposed to be shared terms, and are added to the equality engine for the sake of propagating equalities between shared terms.  The array theory does two non-standard things:
(1) it registers (possibly) non-shared terms as triggers in its `preRegisterInternal` method,
(2) it filters propagations between anything except non-shared arrays in its `eqNotifyTriggerTermEquality` method.

The latter is only necessary because of the former.  It is unnecessary to do *either* of these things. Instead, the array theory should simply add the terms to the equality engine (as non-triggers) during preRegisterInternal.  Note that it additionally correctly adds trigger terms in its `notifySharedTerm` method.

With this commit, the array theory uses the equality engine (wrt propagation) in a standard way.