ANDROID: 16K: Introduce pgsize_migration_inline.h

Introduce inline header to avoid circular dependency. This
will be used in a subsequent patch.

Also take opportunity to do some small noop refactor in
vma_pad_pages() and split_pad_vma() for more robust code.

Bug: 357901498
Change-Id: Ia5f447758d0d07ed3e1429ca1e35dcc0741cc22a
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>