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We are soon moving android-data directory from the current
fixed path (/home/chronos/user/android-data) to a path inside
mounted cryptohome (/home/root/<CryptohomeID>/android-data).
To prepare for that change, we start sending cryptohome ID in
StartArcInstance() D-Bus method.

Since we do not use |socket_path| parameter at all in
session_manager today, we will reuse it to pass the cryptohome ID.

BUG=b:26700652
TEST=trybot
TEST=ARC boots

Review-Url: https://codereview.chromium.org/2023803003
Cr-Commit-Position: refs/heads/master@{#398232}