Refactor GridSizeMigrationTaskV2

The majority of the logic in grid size migration should be implemented
as static functions in the first place since (1) they only runs once
after instantiation and (2) they are executed immediately after they are
instantiated.

This CL removes most of the instance variables from grid size migration
in favor of static methods since the later is more efficient, more
testable and therefore considered better programming practice in
general.

Bug: 256859723
Test: atest GridSizeMigrationUtilTest
Change-Id: Ief4654d79ad5cfd636e6145fdcc9dfe5a39cdf62