Expose getSpi from crypto operations as hidden API.

This makes it possible for platform code to obtain the current SPI
implementation of Cipher, Signature, Mac, and KeyAgreement instances.

The current use case is to access additional information about these
crypto operations when they are backed by Android KeyStore.

NOTE: The getSpi API will not and cannot become public framework API.

Bug: 18088752
Change-Id: If575726d288eebc73ffa3209c316fad071a087fd