Do not modify vector after getting references

We used to obtain a reference to a specific element inside a vector. We
would then modify the vector, invalidating the reference. But we then
used the reference, and passed it to 'assignPointerIds'.

Refactor the code to modify the collection first, and then to proceed
with modifying / reading the elements.

Bug: 179839665
Test: atest inputflinger_tests (on a hwasan build)
Merged-In: I9204b954884e9c83a50babdad5e08a0f6d18ad78
Change-Id: I9204b954884e9c83a50babdad5e08a0f6d18ad78