Fix rounded background for bundles

The last notification and all bundles were merged in the same bucket,
therefore the last notification did not have proper rounded bottom
corners and notifications were missing proper rounded top corners.

Each Bundle now has its own bucket such that it is rounded properly on
top & bottom.

Test: Manual test Buckets now have rounded corners
Bug: b/389839492
Flag: com.android.systemui.notification_bundle_ui
Change-Id: Ie5a8081548c0ea117502ddf4825a1398aa205745