Make isSettlement flag immutable

[liakify]

Expense was meant to be sort of stateless (with the exception of isDeleted) to minimize risk of bugs but along the way became mutable.

Changes:

• Made isSettlement flag immutable

Actually tried to make involvedIds[] immutable but it turned out to be lot more involved (pun not intended) than I thought it would be so didn't do it.

[Aulud]

@liakify Overall, the changes look okay. However, regarding the array involvedIds, any particular reason we decided to use a primitive integer array for it instead of say, a List<Integer>? Using a Collection would make immutability much simpler since the ArrayList class specifies a copy constructor which we can use to quickly copy an ArrayList to mutate before setting it as a final reference in an Expense instance.

Does this also fix the bug from @JohnNzj's PR with serialisation and de-serialisation of JSON expenses involving settlements and deletion of expenses? Currently, the UI is still doing the logic work for rendering the list of settlements but I'm currently doing a refactoring pass on the UI and Activity class to ensure proper separation of concerns - so I'll update you tonight.

[Aulud]

Re-approving since the branch was updated to merge new changes from master at commit d109f5b.