Flow refinement of string conversion node causes inconsistent ATM

[zcai1]

Observation:
After applying the changes in PR #207, calling atypefactory.getAnnotatedType() on a non-string object in string concatenation returns an ATM of <refined annotations, object's real underlying java type>. For example,

"a" + List.of("b")

on the rhs, the ATM of the orignal tree can be <@Peer, List> and the ATM of the converted string may be <@Bottom, String>. However, getAnnotatedType() will return <@Bottom, List> instead.

Expected behavior:
getAnnotatedType() returns an ATM of <refined annotations, String>, and we need an extra helper function to fetch the ATM of the original tree.