arc: bluetooth: Always acquire a NotifySession

arc: bluetooth: Always acquire a NotifySession

If Arc++ needs to keep receiving notifications it needs to
unconditionally acquire a NotifySession. The Bluetooth API
uses a ref-counting model to write to the descriptor. If
other clients drop their NotifySessions, notifications will stop.

BUG=632476
TEST=Build

Review-Url: https://codereview.chromium.org/2189183002
Cr-Commit-Position: refs/heads/master@{#408692}
(cherry picked from commit 3a796de6b43352fb756aa1c9c24a8bc5fc481553)

[rkc, 2016-08-23 01:51:13]

Committed patchset #1 (id:1) to pending queue manually as
23d4630907a678fca6cecb48f48e6a89bce694d0.