Handle all BundleHeader interactions via ExpandableNotificationRow

Having a click handler on BundleHeader but actual long-click handling on the ExpandableNotificationRow led to both of these nodes being focusable accessibility nodes, which in turn leads to confusing behavior where it's not clear which node is selected.

This change also removes all semantics from the BundleHeader so that it does not receive any focus, and in exchange explicitly sets the content description (to a string combining the bundle title and the number of notifications) when initializing ExpandableNotificationRow's accessibility node info.

Fixes: 425576022
Test: performed click, long-click, and each action (expand/collapse/dismiss) on the bundle header using both TalkBack and Voice Access
Flag: com.android.systemui.notification_bundle_ui
Change-Id: I0a5ba13bcb0dd2f24d01e76a144df73a44c20d6d