Solve the composer service NULL pointer crash

The callback functions are set nullptr in ComposerClient::~ComposerClient()
./composer/2.1/default/ComposerClient.cpp:160:    mHal.enableCallback(false)

However, the callbacks are still used in hwc2on1adaptor, it will cause segment fault.

Bug: 64650599

Test: adb shell stop

Change-Id: I92e6a6b42418e987a7ead041f2d95c194b6a16e8
Signed-off-by: Randy Xu <randy.xu@intel.com>
Signed-off-by: Qiming Shi <qiming.shi@intel.com>
Signed-off-by: Yong Yao <yong.yao@intel.com>
Signed-off-by: Gaurav Sarode <sgaurav@google.com>