Bluetooth: Fix to avoid native crash because sJniCallbacksField null

    A case, where BT ON/OFF is running in the loop(monkey test).
    In the monkey test, BT is enabled immediately BT enters off state, but before BT process completely died.
    BT will crash because of null value of sJniCallbacksField in com.android.bluetooth.btservice.AdapterService.initNative.
    Check null to avoid crash.

CRs-Fixed: 961597

Change-Id: I2a34298f40ac15caa225be10407a6f8907e1b042
Signed-off-by: juncao <juncao@codeaurora.org>