Make ENR getViewAtPosition calculation recursive

The guts of notifications within bundled groups would never open for
a long press because the calculation would always return the group ENR,
never its children. With this fix the getViewAtPosition from ENR is
recursively called such that the actual child can be found.

Bug: 389839492
Test: Opened guts for nested notification
Flag: com.android.systemui.notification_bundle_ui
Change-Id: I0a42621a9a548b3d952a54468d1d771907775e86