Keystore 2.0: Rename legacy_migrator to importer.

This refactor serves the disambiguation between the import of legacy
blobs into the sqlite3 database and the migration of keys between
namespaces, which becomes more promient in the light of shared uid
migration.

Test: No new test required because no functionality was changed.
Change-Id: I0612d0731083548a196911712f6c0adbbc9a5e9a
9 files changed