GATT Client : Fix subscription state mis-matched

GATT Client : Fix subscription state mis-matched

[Problem] When BLE is not paired and StartNotify is called via
 bluetooth_gatt_watch_characteristics(), pairing procedure is started.
 We use 5 seconds dbus timeout in that API. So during pairing,
 bluetooth_gatt_watch_characteristics() call is failed.
 But after pairing is done, bluez starts notify procedure.
 So that, subscription is done but application side doesn't know it
 and callback is not set.
[Cause & Measure] Too short timeout is used. ATT transaction timeout is
 30 seconds. If it is over, ATT connection will be dropped. So until
 that, this API shall not be returned. If application needs to be
 returned immediately, it should ensure that both devices are paired
 before bluetooth_gatt_watch_characteristics() is called.
[Checking Method] Try to pair using
 bluetooth_gatt_watch_characteristics() -> Accept pairing request after
 5 seconds.

Change-Id: I3d34f85d0e45cb08039dad843190091e2eeeecc8