LE: Ignore non-volatile GATT service cache for non-bonded devices

If a device is not bonded, the non-volatile GATT service cache should
not be used. Only bonded devices have the capability to invalidate the
cache using the service changed indication.

Bug: 18571592
Change-Id: Ia75aa14f1c862a18237e78d24b63c22cf1dc82a8