Clean up index assignments and comparators

* The main "fix" here is to set the stable index on the summary of a notification, which ensures that if we prune the group, we won't lose the ordering.
* Additionally, I made a functional change here to ignore the stable index when a notification moves between sections (which is super rare anyway) because that value would enforce an incorrect order.

Test: atest ShadeListBuilderTest
Bug: 237216329
Change-Id: Ia36f652478a9efdad71ba651640a12da5fa1345b