identity: Add multi-document presentation support.

This new PresentationSession interface enables an application to do a
multi-document presentation, something which isn't possible with the
existing API. As a practical example of this consider presenting both
your Mobile Driving License and your Vaccination Certificate in a single
transaction.

Also update the documentation for IdentityCredential to clarify that
the same AuthKey is used for multiple getEntries() calls on the same
credential.

Also deprecate existing IdentityCredential.getEntries() method and
related methods and classes.

Bug: 197965513
Test: New CTS tests and new screen in CtsVerifier
Change-Id: I74534969143882552407917a82f44d43da12711c