Fix missing header text in notification groups

Fixes a case where notification header text could go missing
if a notification view was recycled and previously had a header text.

Reapplying only hid the text without clearing it, so the extraction
logic thought it was still there and hid the text for the children
even though it was not showing for the parent.

Change-Id: I3f96e1e7bebb2f815020d278ad13b2b5d948e63c
Fixes: 29915184