Update WifiKeystore and WifiMigration behavior

when the supplicant cannot access WifiBlobstore.

On vendor partitions that are <= T, the supplicant
will expect certs to be in the Legacy Keystore
database. This means that:
 - New certs should be stored in Legacy Keystore
 - Certificates should not be migrated out of
   Legacy Keystore on bootup.

Bug: 353140706
Flag: EXEMPT bugfix
Test: atest WifiKeystoreTest WifiMigrationTest
Test: Manual test - verify that the certs are
      stored in the expected database on a V
      device and a V sys + T vend device
Change-Id: Ic7c49b5b3d1ad310b23f201d20c45b4fee142d22