Simplify ExecutionCallback

This CL simplifies ExecutionCallback by (1) merging the functionality of
CallbackBase into ExecutionCallback, (2) restricts permission to methods
that are only used internally in ExecutionCallback, and (3) simplifies
some of the documentation.

Bug: 118624080
Test: mma
Test: atest NeuralNetworksTest_static
Test: atest VtsHalNeuralnetworksV1_0TargetTest (with sample-all)
Test: atest VtsHalNeuralnetworksV1_1TargetTest (with sample-all)
Test: atest VtsHalNeuralnetworksV1_2TargetTest (with sample-all)
Change-Id: I8e0cb6cec0e71350a742be0ec05fb95a288dd78d