Update documentation for AInputEvent_toJava

As AInputEvent_toJava makes JNI calls, the caller must ensure there are
no pending JNI exceptions as it may result in undefined behaviour. This
needs to be explicitly documented.
Refer: https://developer.android.com/training/articles/perf-jni#exceptions

Bug: 324375527
Test: atest MotionEventTest KeyEventTest
Change-Id: If9f40ba1ee2cb091514fb3baae28f2ca7520727e