Fix some issues in notification bucketing

1 - fix the isHighPriority method to not boost
notifications that users have manually demoted
2 - include the isHighPriority field in the notif sort
so that the divider can be drawn in the right place

Test: atest
Fixes: 133134820
Bug: 129772718
Change-Id: I2c8f6d81e17b50a5be3ffe498536f5f3e59a0dc9