Rename window decor view holders

There were multiple types of handles/headers before, so more descriptive
names were needed to differentiate between them. Now that there's only
one App Handle and one App Header it makes sense to rename them for
simplicity

Flag: EXEMPT refactor
Bug: None
Test: None
Change-Id: I37c01d7a82fdd25a4fcc2dbab6a11a735be0242b