Convert migrateLegacyKeystoreToWifiBlobstore to

an asynchronous method.

API council suggested that the migration can
be a heavy operation, so it is best handled
in the background.

Bug: 358618927
Flag: android.net.wifi.flags.legacy_keystore_to_wifi_blobstore_migration_read_only
Test: atest WifiMigrationTest
Change-Id: I1a61eda93e9f3e2cf0081326777d20a618acb456