neuralnetworks: fix out ParcelFileDescriptor

Refactored "out @nullable ParacelFileDescriptor" into
FencedExecutionResult. It works well with NDK backend, but not with Java
backend. To avoid future incompatibility, AIDL compiler will forbid "out
ParcelFileDescriptor".

This change moves the out parameter to its own parcelable type,
FencedExecutionResult, along with IFencedExecutionCallback.

Bug: 181194872
Test: atest --test-mapping packages/modules/NeuralNetworks
Change-Id: I8e628a7b96d81338fbe59ce8c0b849f98c00536c