arc: Add P256 GEEK Root

Remote Provisioning needs Google Endpoint
Encryption Keychain to verify that the chain is
coming from Google. Currently, the code supports
X25519.

Since ChromeOS would be using the P-256 key, we need
to modify and change the GEEK for P-256 and the
associated parsing method. This CL changes those 2 things.

BUG=b:369207052
TEST=emerge-brya chromeos-base/arc-keymint
TEST=cros_workon_make --board=brya chromeos-base/arc-keymint --test

Change-Id: I4df4e660fc9d0ceac47a6562314b86792ca623fe
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/keymaster/+/5894192
Reviewed-by: Yao Li <yaohuali@google.com>
Tested-by: Vaibhav Raheja <vraheja@chromium.org>
Reviewed-by: Seth Moore <sethmo@google.com>
Commit-Queue: Vaibhav Raheja <vraheja@chromium.org>