[rkpd_client] Add Error type to rkpd_client

This makes rkpd_client independent of keystore2 and facilitates
the extraction of rkpd_client as a standalone library later.

Test: atest keystore2_test
Bug: 241428146
Change-Id: I3bcf0afdb587b2e95bd9a970631c29696f57ed4f
4 files changed