Fix symmetric key generation in strongbox

The strongbox flag was not passed to keystore by
AndroidKeyStoreKeyGeneratorSpi. As a result keys, that were supposed to
be generated in strongbox would silently be generated in TEE.

Test: There is no reliable way to test this other than instrumenting or
      debugging the strongbox implementation. This was done by the
      author of this patch.
Bug: 109769728
Change-Id: I8a08838440030fab7b774762c3d6af0d3b6a4ad8