arc: Convert app availability from OPTIONAL to AVAILABLE

OPTIONAL and AVAILABLE are synonymous. In this change we're updating the
policy_bridge to convert OPTIONAL to AVAILABLE so that DPC will only
deal with one type.

BUG=b:244605914
TEST=run_unit_tests --gtest_filter='ArcPolicyBridgeTest*'
TEST=deploy to DUT and run arc.Unicorn* tests

Change-Id: I5b41a9f8d507078a846ce8332b9dc6db5f46f97c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4390603
Auto-Submit: Muhammad Hasan Khan <mhasank@chromium.org>
Reviewed-by: Yao Li <yaohuali@google.com>
Commit-Queue: Yao Li <yaohuali@google.com>
Cr-Commit-Position: refs/heads/main@{#1124995}