Keystore 2.0 SPI: Fix NPE in getUniqueAliases.

getUniqueAliases may return a null if an error occurred. This would lead
to a NPE in engineAliases.

This patch makes getUniqueAliases return an empty HashSet instead.

Test: atest KeystoreTests

Change-Id: I387d90ea851a8b9c18bb2b20d1a0bfc1ab76c99f