GATT: Thread-safe ClientHandler invocation

Handler could be null when do cleanup in main thread, then NPE
will happen if the ClientHandler thread is still processing
messages.

Bug: 66544093
Test: Enable Bluetooth and search Bluetooth devices, turn off the
Bluetooth and repeat above steps.

Change-Id: Ie8a0d71236bd96e31b5524bcc4690796ad34d68f
Signed-off-by: xutianguo <xutianguo@xiaomi.com>