Don't always mark array types as precise.

Array types are always final, as they cannot be sub-classed, they may be
assigned by types that aren't themselves, e.g. Object[] may be assigned a
String[]. Introduce CannotBeAssignedFromOtherTypes to capture this property
and use in the verifier instead of IsFinal to mark things as precise.

Change-Id: I34debd3164ca1b876be94b6722457cee6e1e9bd3