Change NN canonical timings to nanoseconds -- hal

A sibling CL to this CL changes the definition of nn::TimePoint to the
same type as std::chrono::steady_clock::time_point but has changed the
underlying duration representation to use uint64_t. That sibling CL also
renames nn::OptionalTimeoutDuration to nn::OptionalDuration, and changes
the definition to the same type as std::nanoseconds except the
underlying duration representation now uses uint64_t.

This CL makes changes to the NN HAL utility code in response to the
changes in the sibling CL.

Bug: 174297663
Test: mma
Test: NeuralNetworksTest_static
Change-Id: If44d9aefadb2c78b632ff289b5ff5a49f766525c