Fix asynchronous task implementation in BLE Gatt Service

Fixes the asynchronous task implementation in
BluetoothGattService class. This issue is seen with Motorolla's
BLE android  application.

Change-Id: I0c167145b4f896908abb301859b76fe70e8fbea4
CRs-fixed: 471525
Fix Deadlock issue while closing gatt service with notification on

Removed the syncronized keyword from the deregister characteristics
watcher method. Each profile would create a new BluetoothGattService
for doing gatt operations even if it means the same service on the
same remote device. Therefore, two or more threads cannot access
deregister watcher in BluetoothGattService at the same time.
CRs-fixed: 490897

Change-Id: I7fc8728a8c9c3769c7763ac8fc01ba7ee2f64730