Removing notification headers as valid magnetic boundaries.

By design, views that can't be magnetically dragged (such as headers)
should not be pulled when a neighbor magnetic notification is dragged.

Test: manual. Verified notification headers (e.g., "Silent") do not move
  when dragging a neighbor notification.
Flag: com.android.systemui.magnetic_notification_swipes
Bug: 413672316
Change-Id: Id92e0a91fc5bb9cb5cb70ce4875bc8f5e3f8f7d3