arc/bluetooth: plumb adapter discoverable timeout property

Instead of hardcoding this to 120, we'll pull in the real timeout
reported by BlueZ.

BUG=b:29617227
TEST=request properties and check for the timeout

Review-Url: https://codereview.chromium.org/2083423004
Cr-Commit-Position: refs/heads/master@{#402302}
3 files changed