floss: Refactor build for bt_common tests

Add futures library to Cargo.toml and refactor cxxbridge build to
include the fake keystore required for tests in bt_common.

Bug: 232134607
Tag: #floss
Test: ./floss/build/build-in-podman.py and emerge-zork floss
Change-Id: I4e5c5e43d38587db902d77f4c20d64255e986371