Implement schema migration to another type in framework.

Changes included:
*05b7d39:Change schema version from per AppSearchSchema to overall.
*effe024:Support schema migration to another type.
*1be54dc:Minor fix for where we set version in schema.

Bug: 182620003
Test: AppSearchSchemaMigrationCtsTest
Change-Id: Ifabfaa65c32fc3684d7fc7a09dd021671ff3252d
5 files changed