[Bluetooth][BugFix]:Fix Bluetooth unable to enable

MIUI-5324

Test apk test Bluetooth enable and disable, In rare scenarios,mBluetooth not set to null,and when apk enable Bluetooth, it will send MESSAGE_RESTART_BLUETOOTH_SERVICE
Continuously and unable to enable BT

solution: when enable BT and Reach maximum retry to restart Bluetooth, set mBluetooth null, it can enble BT successfully

Signed-off-by: sunxiang <sunxiang@xiaomi.com>
Change-Id: Ic88886042fa550825e30196c37f36c61f7c2fb6b