ANDROID: crypto: heh - factor out poly_hash algorithm

ANDROID: crypto: heh - factor out poly_hash algorithm

Factor most of poly_hash() out into its own keyed hash algorithm so that
optimized architecture-specific implementations of it will be possible.

For now we call poly_hash through the shash API, since HEH already had
an example of using shash for another algorithm (CMAC), and we will not
be adding any poly_hash implementations that require ahash yet.  We can
however switch to ahash later if it becomes useful.

Bug: 32508661
Signed-off-by: Eric Biggers <ebiggers@google.com>
Change-Id: I8de54ddcecd1d7fa6e9842a09506a08129bae0b6