aconfig: add whole table serialization test

1, Add whole table serialization test to also lock down how the buckets
are serialized. Before this change, we are only locking down the header
and nodes serialization.

2, Switch over to the absolute offset in the file instead of relative
offset. Before we are using relative offset. For example, if a bucket is
Some(10), it means that it is pointing to 10 bytes over the start of the
node region of the table. Now it will be the absolute byte offset with
respect to the start of the table.

Bug: b/312243587
Test: atest aconfig.test
Change-Id: If7abc8c6b6687c0bc0c40bbfc6afbe0e46ece770