Create separate dimensions for the notificaitons and notification rows.

Use notification heights on layouts internal to the notificaitons.
Use row heights where padding needs to be protected, on row layout decisions.
Clean up values-xhdpi/dimens.xml since there is no longer a values-hdpi/dimens.xml.

Bug: 6475098
Bug: 6489095
Change-Id: I44750574c6e23336585a432a38c5f1ba183b6cbc