Expose getCurrentSpi from crypto operations as hidden API.

050e672a exposed the existing getSpi
method of Cipher, Signature, Mac, and KeyAgreement as hidden API.
Unfortunately, the getSpi method creates an SPI instance if one is
not yet set. This changes the state of the crypto operation and does
does not lend itself well to being used for read-only querying of
the SPI from a crypto operation.

This CL addresses the issue by adding a getCurrentSpi hidden API to
these crypto operations. getCurrentSpi simply returns the current SPI
instance, if any, and does not modify the state of the crypto
operation.

A follow-up CL will revert 050e672a
which will no longer be needed. This is not reverted here to avoid
breaking the build.

Bug: 18088752
Change-Id: Ifc0b0db1bcbc7447353254f0e2bcf8962cd3919e