Modify AttestationUtils to use public Keystore API

AttestationUtils calls directly into keystore1 to generate ID
attesations.  This needs to change prior to keystore2 being enabled
and keystore1 deleted.  This CL changes the AttestationUtils to use
the public API (and one SystemAPI method) to generate ID attestations,
allowing the lower layers to handle the transition between keystore1
and keystore2.

Test: CtsKeystoreTestCases
Change-Id: I64a230b9983cc90767a60d6e7cf2abcf5dfb0108