Fix a use-of-uninitialized-value warning.

Caught by clang's static analyzer:
frameworks/base/core/jni/android_hardware_SoundTrigger.cpp:660:20:
warning: Assigned value is garbage or undefined
        nHandle[0] = (jint)handle;

Given that we return an error code anyway, we can just set it to 0 and
move on.

Bug: None
Test: Still builds; static analyzer doesn't complain.
Change-Id: I9a5be562a0d96fd6c48bb98eca0867fc754bba63