dbus: bluetooth: Add GATT option string to match blueZ API change

The BlueZ CL below added option dict to ReadValue and WriteValue.
This CLs add string constant for the key of this option dict.

http://git.kernel.org/cgit/bluetooth/bluez.git/commit/?id=93b64d9

BUG=chromium:614903
TEST=Can read characteristic value from Pixel C keyboard

Change-Id: I146b04c738fd75703eedd26300fd855b07a414be
Signed-off-by: Puthikorn Voravootivat <puthik@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/347459
Reviewed-by: Dan Erat <derat@chromium.org>
1 file changed