[views_ax] Introduce ViewAccessibility setters for PosInSet and SetSize

This CL introduces

* ViewAccessibility::SetPosInSet
* ViewAccessibility::SetSetSize
* ViewAccessibility::ClearPosInSet
* ViewAccessibility::ClearSetSize

We'll replace the callers of OverridePosInSet and ClearPosInSetOverride in the
next CL, and remove those overrides quickly after.

Previously, OverridePosInSet and ClearPosInSetOverride took care of not only the PosInSet attribute, but also the SetSize. There could
be valid cases where the set size changes but the PosInSet doesn't,
so we prefer to split those setters into atomic setters. It will also
make it easier to add automatic event generation per attribute if
needed.

This results in the same behavior, and eventually we will remove the
override functions.

Bug: 325137417
Change-Id: I17bde4a528b86b8ca70df42ab39fe4116b021e78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5304030


Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1262110}