Add the chrome.bluetoothLowEnergy.setAdvertisingInterval API.

This API allows us to set the advertising interval on a device. This API is
restricted to an auto-launched app running in Kiosk mode since it can alter
the state of a device in a way that would effect other users.

Reviews requested,
xiyuan@ - overall
rdevlin.cronin@ - //extensions/OWNERS
ortuno@ -- //device/bluetooth/OWNERS
mpearson@ -- histograms

R=mpearson@chromium.org, ortuno@chromium.org, rdevlin.cronin@chromium.org, xiyuan@chromium.org
BUG=637231

Review-Url: https://codereview.chromium.org/2353133005
Cr-Commit-Position: refs/heads/master@{#420254}
27 files changed