[cbor] Refactor building the first part of bcc with ciborium

This cl replaces the manual CBOR serialization of the firsr part
of bcc (root public key + root certificate) by the crate ciborium.

Test: Run microdroid.
Test: atest microdroid_manager_test
Bug: 268322533
Change-Id: Ie75a11ff9ca75ea35b7c8b029e7d086d010afded
2 files changed